feat(backend-tests): [sep-12] write e2e tests for pre-signed url uplo…

[heymide]

Close: #561

Summary of Changes

1. Updated backend/src/api/controllers/sep12.controller.test.ts to add mocks for:
   • storageProvider.service (mock S3-like storage service)
   • uploadStore.service (in-memory upload record store)
   • config/env (SEP-12 max file size configuration)
2. Added tests for getUploadUrl method :
   • Happy path: successfully returns a pre-signed URL
   • Missing required parameters returns 400
   • Invalid content-type returns 400
   • Oversized file (exceeds max allowed size) returns 400
3. Added tests for confirmUpload method :
   • Happy path: successfully confirms upload and updates status to COMPLETED
   • Missing required parameters returns 400
   • Non-existent upload record returns 404
   • Account mismatch returns 403
   • File not found in storage returns 422
     The network issue is preventing us from installing dependencies right now, but the test implementation itself is complete and follows the project's existing test conventions!

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[heymide]

Done, Close: #677

[Ceejaytech25]

Nice implementation, LGTM!