fix(api): prevent MultipartFile binding errors in StampController (#4331)

# Description of Changes

- **What was changed**
- Added a Spring `@InitBinder` in `StampController` that registers a
`PropertyEditorSupport` for `MultipartFile` to safely handle text inputs
by setting the value to `null`.
- Replaced multiple `switch` statements with modern Java **switch
expressions**:
    - Margin selection (`customMargin`) now uses a concise expression.
- Font selection (`alphabet` → font resource path) rewritten as an
expression.
- Position calculations (`calculatePositionX` / `calculatePositionY`)
now return via switch expressions with `yield`.
- Minor readability and maintainability improvements without changing
public API or behavior (besides the binding fix).

- **Why the change was made**
- The `@InitBinder` prevents conversion/binding issues when Spring
attempts to map string form fields to `MultipartFile`, which could cause
exceptions or unexpected behavior in multipart/form-data requests.
- Using switch expressions reduces boilerplate, clarifies intent, and
makes the control flow safer and more maintainable.

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

app/core/src/main/java/stirling/software/SPDF/controller/api/misc/StampController.java

@@ -2,6 +2,7 @@ package stirling.software.SPDF.controller.api.misc;
 
 import java.awt.*;
 import java.awt.image.BufferedImage;
+import java.beans.PropertyEditorSupport;
 import java.io.File;
 import java.io.FileOutputStream;
 import java.io.IOException;
@@ -25,6 +26,8 @@ import org.apache.pdfbox.pdmodel.graphics.state.PDExtendedGraphicsState;
 import org.apache.pdfbox.util.Matrix;
 import org.springframework.core.io.ClassPathResource;
 import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.WebDataBinder;
+import org.springframework.web.bind.annotation.InitBinder;
 import org.springframework.web.bind.annotation.ModelAttribute;
 import org.springframework.web.bind.annotation.PostMapping;
 import org.springframework.web.bind.annotation.RequestMapping;
@@ -52,6 +55,24 @@ public class StampController {
     private final CustomPDFDocumentFactory pdfDocumentFactory;
     private final TempFileManager tempFileManager;
 
+    /**
+     * Initialize data binder for multipart file uploads.
+     * This method registers a custom editor for MultipartFile to handle file uploads.
+     * It sets the MultipartFile to null if the uploaded file is empty.
+     * This is necessary to avoid binding errors when the file is not present.
+     */
+    @InitBinder
+    public void initBinder(WebDataBinder binder) {
+        binder.registerCustomEditor(
+                MultipartFile.class,
+                new PropertyEditorSupport() {
+                    @Override
+                    public void setAsText(String text) throws IllegalArgumentException {
+                        setValue(null);
+                    }
+                });
+    }
+
     @PostMapping(consumes = "multipart/form-data", value = "/add-stamp")
     @Operation(
             summary = "Add stamp to a PDF file",
@@ -91,25 +112,14 @@ public class StampController {
         float overrideY = request.getOverrideY(); // New field for Y override
 
         String customColor = request.getCustomColor();
-        float marginFactor;
+        float marginFactor =
-
+                switch (request.getCustomMargin().toLowerCase()) {
-        switch (request.getCustomMargin().toLowerCase()) {
+                    case "small" -> 0.02f;
-            case "small":
+                    case "medium" -> 0.035f;
-                marginFactor = 0.02f;
+                    case "large" -> 0.05f;
-                break;
+                    case "x-large" -> 0.075f;
-            case "medium":
+                    default -> 0.035f;
-                marginFactor = 0.035f;
+                };
-                break;
-            case "large":
-                marginFactor = 0.05f;
-                break;
-            case "x-large":
-                marginFactor = 0.075f;
-                break;
-            default:
-                marginFactor = 0.035f;
-                break;
-        }
 
         // Load the input PDF
         PDDocument document = pdfDocumentFactory.load(pdfFile);
@@ -185,27 +195,16 @@ public class StampController {
             throws IOException {
         String resourceDir = "";
         PDFont font = new PDType1Font(Standard14Fonts.FontName.HELVETICA);
-        switch (alphabet) {
+        resourceDir =
-            case "arabic":
+                switch (alphabet) {
-                resourceDir = "static/fonts/NotoSansArabic-Regular.ttf";
+                    case "arabic" -> "static/fonts/NotoSansArabic-Regular.ttf";
-                break;
+                    case "japanese" -> "static/fonts/Meiryo.ttf";
-            case "japanese":
+                    case "korean" -> "static/fonts/malgun.ttf";
-                resourceDir = "static/fonts/Meiryo.ttf";
+                    case "chinese" -> "static/fonts/SimSun.ttf";
-                break;
+                    case "thai" -> "static/fonts/NotoSansThai-Regular.ttf";
-            case "korean":
+                    case "roman" -> "static/fonts/NotoSans-Regular.ttf";
-                resourceDir = "static/fonts/malgun.ttf";
+                    default -> "static/fonts/NotoSans-Regular.ttf";
-                break;
+                };
-            case "chinese":
-                resourceDir = "static/fonts/SimSun.ttf";
-                break;
-            case "thai":
-                resourceDir = "static/fonts/NotoSansThai-Regular.ttf";
-                break;
-            case "roman":
-            default:
-                resourceDir = "static/fonts/NotoSans-Regular.ttf";
-                break;
-        }
 
         if (!"".equals(resourceDir)) {
             ClassPathResource classPathResource = new ClassPathResource(resourceDir);
@@ -327,30 +326,30 @@ public class StampController {
             throws IOException {
         float actualWidth =
                 (text != null) ? calculateTextWidth(text, font, fontSize) : contentWidth;
-        switch (position % 3) {
+        return switch (position % 3) {
             case 1: // Left
-                return pageSize.getLowerLeftX() + margin;
+                yield pageSize.getLowerLeftX() + margin;
             case 2: // Center
-                return (pageSize.getWidth() - actualWidth) / 2;
+                yield (pageSize.getWidth() - actualWidth) / 2;
             case 0: // Right
-                return pageSize.getUpperRightX() - actualWidth - margin;
+                yield pageSize.getUpperRightX() - actualWidth - margin;
             default:
-                return 0;
+                yield 0;
-        }
+        };
     }
 
     private float calculatePositionY(
             PDRectangle pageSize, int position, float height, float margin) {
-        switch ((position - 1) / 3) {
+        return switch ((position - 1) / 3) {
             case 0: // Top
-                return pageSize.getUpperRightY() - height - margin;
+                yield pageSize.getUpperRightY() - height - margin;
             case 1: // Middle
-                return (pageSize.getHeight() - height) / 2;
+                yield (pageSize.getHeight() - height) / 2;
             case 2: // Bottom
-                return pageSize.getLowerLeftY() + margin;
+                yield pageSize.getLowerLeftY() + margin;
             default:
-                return 0;
+                yield 0;
-        }
+        };
     }
 
     private float calculateTextWidth(String text, PDFont font, float fontSize) throws IOException {