Multi module refactor (#3640)

# Description of Changes

Migrated Stirling PDF to a multi-module structure:

* Introduced new `:stirling-pdf` module
* Moved all the core logic and features of Stirling PDF into
`:stirling-pdf`
* Updated paths of jobs and scripts

---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/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/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] 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/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/DeveloperGuide.md#6-testing)
for more details.

.github/workflows/check_properties.yml

@@ -4,7 +4,7 @@ on:
   pull_request_target:
     types: [opened, synchronize, reopened]
     paths:
-      - "src/main/resources/messages_*.properties"
+      - "stirling-pdf/src/main/resources/messages_*.properties"
 
 permissions:
   contents: read # Allow read access to repository content
@@ -61,7 +61,20 @@ jobs:
         run: |
           echo "Fetching PR changed files..."
           echo "Getting list of changed files from PR..."
-          gh pr view ${{ steps.get-pr-data.outputs.pr_number }} --json files -q ".files[].path" | grep -E '^src/main/resources/messages_[a-zA-Z_]{2}_[a-zA-Z_]{2,7}\.properties$' > changed_files.txt # Filter only matching property files
+                   # Check if PR number exists
+            if [ -z "${{ steps.get-pr-data.outputs.pr_number }}" ]; then
+              echo "Error: PR number is empty"
+              exit 1
+            fi
+            # Get changed files and filter for properties files, handle case where no matches are found
+            gh pr view ${{ steps.get-pr-data.outputs.pr_number }} --json files -q ".files[].path" | grep -E '^stirling-pdf/src/main/resources/messages_[a-zA-Z_]{2}_[a-zA-Z_]{2,7}\.properties$' > changed_files.txt || echo "No matching properties files found in PR"
+            # Check if any files were found
+            if [ ! -s changed_files.txt ]; then
+              echo "No properties files changed in this PR"
+              echo "Workflow will exit early as no relevant files to check"
+              exit 0
+            fi
+            echo "Found $(wc -l < changed_files.txt) matching properties files"
 
       - name: Determine reference file test
         id: determine-file
@@ -103,7 +116,7 @@ jobs:
             // Filter for relevant files based on the PR changes
             const changedFiles = files
               .map(file => file.filename)
-              .filter(file => /^src\/main\/resources\/messages_[a-zA-Z_]{2}_[a-zA-Z_]{2,7}\.properties$/.test(file));
+              .filter(file => /^stirling-pdf\src\/main\/resources\/messages_[a-zA-Z_]{2}_[a-zA-Z_]{2,7}\.properties$/.test(file));
 
             console.log("Changed files:", changedFiles);
 
@@ -141,12 +154,12 @@ jobs:
 
             // Determine reference file
             let referenceFilePath;
-            if (changedFiles.includes("src/main/resources/messages_en_GB.properties")) {
+            if (changedFiles.includes("stirling-pdf/src/main/resources/messages_en_GB.properties")) {
               console.log("Using PR branch reference file.");
               const { data: fileContent } = await github.rest.repos.getContent({
                 owner: prRepoOwner,
                 repo: prRepoName,
-                path: "src/main/resources/messages_en_GB.properties",
+                path: "stirling-pdf/src/main/resources/messages_en_GB.properties",
                 ref: branch,
               });
 
@@ -158,7 +171,7 @@ jobs:
               const { data: fileContent } = await github.rest.repos.getContent({
                 owner: repoOwner,
                 repo: repoName,
-                path: "src/main/resources/messages_en_GB.properties",
+                path: "stirling-pdf/src/main/resources/messages_en_GB.properties",
                 ref: "main",
               });