Tests: Add unit test coverage for private uploads, signed URLs, and privacy export

[noruzzamans]

Summary

This pull request enhances the unit test coverage of S3_Uploads by adding dedicated tests for private uploads, presigned URLs, ACL modifications, S3 location parsing, resource hints, and GDPR personal data exports in tests/test-s3-uploads.php:

• S3 location resolution (test_get_s3_location_for_url, test_get_s3_location_for_path): Verifies canonical S3 URLs, upload directory base URLs, s3:// stream paths, and proper null handling for external URLs or non-S3 paths.
• Resource hints DNS prefetch (test_wp_filter_resource_hints): Verifies that the S3 bucket URL is added to dns-prefetch relation hints, and ensures other relation types remain untouched.
• Private attachment detection (test_is_private_attachment): Validates default behavior (false) and filtering via the s3_uploads_is_attachment_private hook.
• Presigned URLs for private attachments (test_add_s3_signed_params_to_attachment_url): Verifies that public attachments remain untouched, private attachments receive signed S3 parameters (X-Amz-Signature), previous query parameters are stripped to prevent signature corruption, and the s3_uploads_private_attachment_url_expiry filter is respected.
• Image src and srcset signing (test_add_s3_signed_params_to_attachment_image_src, test_add_s3_signed_params_to_attachment_image_srcset): Verifies signed URLs across wp_get_attachment_image_src and responsive wp_calculate_image_srcset sources while handling empty or invalid post IDs.
• Setting attachment files ACL (test_set_attachment_files_acl): Tests batch ACL updates for attachment files and verifies that the s3_uploads_set_attachment_files_acl action fires with the correct attachment ID and ACL parameter.
• Automatic private ACL on metadata generation (test_set_attachment_private_on_generate_attachment_metadata): Verifies that when an attachment is marked private, metadata generation automatically triggers private ACL updates for the generated image sizes.
• GDPR Personal Data Export (test_privacy_personal_data_export_dir_and_file_move): Validates that s3:// exports directories are properly rerouted to a local temp folder (set_wp_privacy_exports_dir), and the created zip archive is securely moved to the S3 exports directory (move_temp_personal_data_to_s3) before the local temp file is unlinked.

Test plan

• Ran ./tests/run-tests.sh: all 37 tests pass with 107 assertions (increased from 27 tests and 66 assertions).
• Psalm static analysis runs with 0 errors (No errors found!).
• Tested against local MinIO S3 object storage container.