Add server-side UploadCompletionListener and return UploadInfo from process()

[tomdesair]

Description

This pull request introduces a server-side upload completion callback mechanism and updates TusFileUploadService.process(...) to return the processed UploadInfo.

Summary of Changes

1. UploadCompletionListener Functional Interface:
   • Introduced @FunctionalInterface public interface UploadCompletionListener with void onUploadComplete(UploadInfo uploadInfo, TusFileUploadService tusFileUploadService).
   • Added registration methods withUploadCompletionListener(UploadCompletionListener) and addUploadCompletionListener(UploadCompletionListener) to TusFileUploadService.
2. Lock Release Before Callback Execution:
   • Evaluates initial progress state (wasInProgress) and triggers listeners when transitioning to complete.
   • The upload lock is released before listeners are executed, allowing listeners to safely call service.getUploadedBytes(uploadInfo) or service.deleteUpload(uploadInfo) without lock contention or UploadAlreadyLockedException.
   • Listener exceptions are isolated so downstream errors do not abort other listeners or corrupt HTTP responses.
3. process() Return Value & Convenience Overloads:
   • TusFileUploadService.process(...) now returns the created or updated UploadInfo (or null on errors / OPTIONS).
   • Added TusFileUploadService.getUploadedBytes(UploadInfo) and TusFileUploadService.deleteUpload(UploadInfo) convenience methods.
4. Testing:
   • Unit tests in UploadCompletionListenerTest.java (8 unit tests).
   • Multi-backend integration tests in AbstractITTusFileUploadService.java and AbstractITRufhProtocol.java covering Tus PATCH, creation-with-upload, concatenation, and RUFH optimistic POST / PATCH appends across Disk, S3, and Azure Blob storage.
5. Documentation & Changelog:
   • Updated README.md with configuration options, session upload ID association examples (Creation vs. Progress check pattern), listener usage, and single-request lifecycle timing notes.
   • Updated CHANGELOG.md under ## [2.0.0].

Verification

• mvn clean install -q: ✅ PASSED
• Unit and Multi-Backend Integration Tests: ✅ PASSED
• python3 scripts/check-coverage.py --per-file-limit 90: ✅ PASSED (Diff coverage: 97.60%, all files > 90%)
• Code style formatting: ✅ PASSED

[coveralls]

Coverage is 94.609% — feature/upload-completion-listener into master. No base build found for master.