test(backend): Phase 1 - Improve testing infrastructure and add error path tests

## Changes

### pytest-asyncio Integration
- Add pytest-asyncio>=0.21.0 to requirements-dev.txt
- Update pytest.ini with asyncio_mode=auto and fixture loop scope
- Remove custom pytest_pyfunc_call handler from conftest.py
- Add @pytest.mark.asyncio to 21 async test functions

### Error Path Tests
- Create test_downloader_error_paths.py with 19 new tests covering:
  - DownloadStreamControl state management (6 tests)
  - Downloader configuration and initialization (4 tests)
  - DownloadProgress dataclass validation (1 test)
  - Custom exception handling (2 tests)
  - Authentication header generation (3 tests)
  - Session management (3 tests)

### Documentation
- Update backend-testing-improvement-plan.md with Phase 1 completion status

## Test Results
- All 458 service tests pass
- No regressions introduced

Relates to backend testing improvement plan Phase 1

pytest.ini

@@ -4,9 +4,13 @@ testpaths = tests
 python_files = test_*.py
 python_classes = Test*
 python_functions = test_*
-# Register async marker for coroutine-style tests
+# Asyncio configuration
+asyncio_mode = auto
+asyncio_default_fixture_loop_scope = function
+# Register markers
 markers =
     asyncio: execute test within asyncio event loop
     no_settings_dir_isolation: allow tests to use real settings paths
+    integration: integration tests requiring external resources
 # Skip problematic directories to avoid import conflicts
 norecursedirs = .git .tox dist build *.egg __pycache__ py