Support multi-file async job results and ZIP extraction (#3922)

# Description of Changes

This PR introduces multi-file support for asynchronous jobs in the
Stirling PDF backend, enabling jobs to return and manage multiple result
files. Previously, job results were limited to a single file represented
by fileId, originalFileName, and contentType. This change replaces that
legacy structure with a new ResultFile abstraction and expands the
functionality throughout the core system.

ZIP File Support
If a job result is a ZIP file:
It is automatically unpacked using buffered streaming.
Each contained file is stored individually and recorded as a ResultFile.
The original ZIP is deleted after successful extraction.
If ZIP extraction fails, the job result is treated as a single file.


New and Updated API Endpoints

1. GET /api/v1/general/job/{jobId}/result

If the job has multiple files → returns a JSON metadata list.

If the job has a single file → streams the file directly.

Includes UTF-8-safe Content-Disposition headers for filename support.

2. GET /api/v1/general/job/{jobId}/result/files
New endpoint that returns:

```json
{
  "jobId": "123",
  "fileCount": 2,
  "files": [
    {
      "fileId": "abc",
      "fileName": "page1.pdf",
      "contentType": "application/pdf",
      "fileSize": 12345
    },
    ...
  ]
}
```


3. GET /api/v1/general/files/{fileId}/metadata
Returns metadata for a specific file:


4. GET /api/v1/general/files/{fileId}
Downloads a file by fileId, using metadata to determine filename and
content type.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: pixeebot[bot] <104101892+pixeebot[bot]@users.noreply.github.com>

common/src/test/java/stirling/software/common/service/TaskManagerTest.java

@@ -18,6 +18,7 @@ import org.springframework.test.util.ReflectionTestUtils;
 
 import stirling.software.common.model.job.JobResult;
 import stirling.software.common.model.job.JobStats;
+import stirling.software.common.model.job.ResultFile;
 
 class TaskManagerTest {
 
@@ -73,13 +74,17 @@ class TaskManagerTest {
     }
 
     @Test
-    void testSetFileResult() {
+    void testSetFileResult() throws Exception {
         // Arrange
         String jobId = UUID.randomUUID().toString();
         taskManager.createTask(jobId);
         String fileId = "file-id";
         String originalFileName = "test.pdf";
         String contentType = "application/pdf";
+        long fileSize = 1024L;
+
+        // Mock the fileStorage.getFileSize() call
+        when(fileStorage.getFileSize(fileId)).thenReturn(fileSize);
 
         // Act
         taskManager.setFileResult(jobId, fileId, originalFileName, contentType);
@@ -88,9 +93,17 @@ class TaskManagerTest {
         JobResult result = taskManager.getJobResult(jobId);
         assertNotNull(result);
         assertTrue(result.isComplete());
-        assertEquals(fileId, result.getFileId());
-        assertEquals(originalFileName, result.getOriginalFileName());
-        assertEquals(contentType, result.getContentType());
+        assertTrue(result.hasFiles());
+        assertFalse(result.hasMultipleFiles());
+        
+        var resultFiles = result.getAllResultFiles();
+        assertEquals(1, resultFiles.size());
+        
+        ResultFile resultFile = resultFiles.get(0);
+        assertEquals(fileId, resultFile.getFileId());
+        assertEquals(originalFileName, resultFile.getFileName());
+        assertEquals(contentType, resultFile.getContentType());
+        assertEquals(fileSize, resultFile.getFileSize());
         assertNotNull(result.getCompletedAt());
     }
 
@@ -163,8 +176,11 @@ class TaskManagerTest {
     }
 
     @Test
-    void testGetJobStats() {
+    void testGetJobStats() throws Exception {
         // Arrange
+        // Mock fileStorage.getFileSize for file operations
+        when(fileStorage.getFileSize("file-id")).thenReturn(1024L);
+        
         // 1. Create active job
         String activeJobId = "active-job";
         taskManager.createTask(activeJobId);
@@ -216,9 +232,15 @@ class TaskManagerTest {
         LocalDateTime oldTime = LocalDateTime.now().minusHours(1);
         ReflectionTestUtils.setField(oldJob, "completedAt", oldTime);
         ReflectionTestUtils.setField(oldJob, "complete", true);
-        ReflectionTestUtils.setField(oldJob, "fileId", "file-id");
-        ReflectionTestUtils.setField(oldJob, "originalFileName", "test.pdf");
-        ReflectionTestUtils.setField(oldJob, "contentType", "application/pdf");
+        
+        // Create a ResultFile and set it using the new approach
+        ResultFile resultFile = ResultFile.builder()
+                .fileId("file-id")
+                .fileName("test.pdf")
+                .contentType("application/pdf")
+                .fileSize(1024L)
+                .build();
+        ReflectionTestUtils.setField(oldJob, "resultFiles", java.util.List.of(resultFile));
 
         when(fileStorage.deleteFile("file-id")).thenReturn(true);