🧪 Add tests for GET /api/export-excel

[deitaur]

🎯 What: The testing gap addressed
The GET /api/export-excel endpoint lacked testing for its complex stream response containing an Excel file.

📊 Coverage: What scenarios are now tested

• Happy Path: Correct generation of an Excel file with expected rows, columns, and data mapped from a mocked database structure (2-level deep validation parsing actual binary buffer).
• Error Edge Case: Endpoint properly captures errors (such as DB failures) and returns a standard 500 response.
• Empty Data Case: When a user has zero items, the generated file gracefully yields a file with headers but no data rows.

✨ Result: The improvement in test coverage
The critical Excel generation feature is now covered with tests that verify internal correctness, ensuring that future refactoring will confidently preserve functionality without requiring manual QA.

Also cleans up app.request mutations properly and resolves package dependencies for testing locally.

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PR created automatically by Jules for task 3871311524714203752 started by @deitaur

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