feat: comprehensive test infrastructure improvements

Phase 1: Fix immediate test failures
- Fix path handling in files router (strip leading slashes)
- Fix test assertions to match actual API behavior
- Add proper error handling for missing files in download endpoint
- Reload files router in test fixtures to pick up config changes
- Fix upload endpoint test to use query params instead of form data

Phase 2: Improve test reliability
- Standardize error responses: docker_router now uses HTTPException
- Add global exception handlers for validation, Docker errors, and unhandled exceptions
- Fix flaky TOTP replay protection test
- Fix flaky password hash loading test with proper module reload
- Enhanced Docker mock fixtures with error scenarios and factory pattern

Phase 3: Add coverage reporting
- Add pytest-cov with 40% minimum coverage threshold
- Add pyproject.toml with pytest and coverage configuration
- Update test workflow to generate coverage and JUnit XML reports
- Remove -x flag to see all test failures
- Configure asyncio_default_fixture_loop_scope to fix deprecation warning

Results:
- All 144 tests passing (was 137 passed, 5 failed, 2 skipped)
- Test execution time: ~3s locally
- Coverage: 43% (above 40% threshold)
- No skipped tests
- Standardized error handling across all endpoints

dashboard/backend/tests/integration/test_docker.py

@@ -79,9 +79,9 @@ class TestDockerContainerActions:
             headers=admin_headers
         )
 
-        assert response.status_code == 200
+        assert response.status_code == 400
         data = response.json()
-        assert "error" in data
+        assert "detail" in data
 
 
 class TestDockerContainerLogs: