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/pyproject.toml

@@ -0,0 +1,39 @@
+[tool.pytest.ini_options]
+minversion = "7.0"
+testpaths = ["tests"]
+python_files = ["test_*.py"]
+python_classes = ["Test*"]
+python_functions = ["test_*"]
+addopts = [
+    "-v",
+    "--strict-markers",
+    "--tb=short",
+]
+asyncio_mode = "auto"
+asyncio_default_fixture_loop_scope = "function"
+
+[tool.coverage.run]
+source = ["."]
+omit = [
+    "*/tests/*",
+    "*/venv/*",
+    "*/__pycache__/*",
+    "*/site-packages/*",
+]
+
+[tool.coverage.report]
+precision = 2
+show_missing = true
+skip_covered = false
+exclude_lines = [
+    "pragma: no cover",
+    "def __repr__",
+    "raise AssertionError",
+    "raise NotImplementedError",
+    "if __name__ == .__main__.:",
+    "if TYPE_CHECKING:",
+    "@abstractmethod",
+]
+
+[tool.coverage.xml]
+output = "coverage.xml"