test: add firmware service unit tests

[JanZachmann]

Summary

Expand firmware service test coverage to validate file cleanup operations and device service client delegation.

Tests now support parallel execution using mutex lock pattern (refactored from initial implementation).

Tests Added (6 new, 7 total)

clear_data_folder (3 tests)

• ✅ Removes all files in data directory
• ✅ Succeeds with empty directory
• ✅ Preserves subdirectories (only files removed)

load_update (2 tests)

• ✅ Forwards request to device service with correct path
• ✅ Returns error on device service failure

run_update (2 tests)

• ✅ Forwards request to device service
• ✅ Returns error on device service failure

Implementation Details

• Uses mockall to mock DeviceServiceClient for async operations
• Tests validate delegation logic without testing device service internals
• Uses serde_json to construct RunUpdate (private fields)
• Tests ensure directory exists before operations
• Parallel test support: Uses static DATA_FOLDER_LOCK mutex following the PASSWORD_FILE_LOCK pattern for test isolation

Refactoring for Parallel Execution

Initially, clear_data_folder tests had a race condition due to shared temp directory from AppConfig. This has been resolved by:

1. Adding static DATA_FOLDER_LOCK mutex (following password.rs pattern)
2. Adding lock_for_test() method to FirmwareService
3. Updating tests to acquire lock before accessing shared data directory
4. Tests now run safely in parallel without --test-threads=1 flag

Known Limitations

File Upload Not Tested: handle_uploaded_firmware is not tested as it requires mocking actix_multipart::form::tempfile::TempFile, which is complex.

Test Results

# All tests pass in parallel
cargo test --features mock firmware::tests

All 7 tests pass ✅

Impact

• Total backend tests: 54 → 60 (+6)
• Coverage: Firmware file operations & device service delegation
• Phase: 1.3 of backend test plan

Checklist

• All tests pass in parallel
• Code formatted with cargo fmt
• Clippy passes
• BACKEND_TEST_CONCEPT.md updated
• Refactored for parallel test execution

Signed-off-by: Jan Zachmann 50990105+JanZachmann@users.noreply.github.com