Improve test coverage (87% → 95%)

[vladiant]

Summary

Adds 44 new tests across 6 test files to close coverage gaps identified in the existing test suite. No production code changes — only new tests, version bump, and changelog update.

Changes

File	Tests	Covers
test_get_image.py	7	GetImageUseCase.execute() and get_file() — all branches including missing path for original/thumbnail
test_metrics_recording.py	5	_record_upload() and _record_duration() metrics paths, including error swallowing
test_health.py	5	_check_database failure, _check_storage missing directory, degraded health endpoint
test_executor_lifecycle.py	5	async_shutdown_executor (no-op, normal, timeout/force-cancel), executor singleton, shutdown_executor
test_observability.py	8	MetricsMiddleware (skip /health and /metrics, record normal/4xx/5xx), setup_tracing, setup_metrics, instrument_logging
test_config_and_main.py	14	Settings.database_url (including special char encoding), build_engine/build_session_factory, create_app ± CORS, _is_otel_enabled/_is_cors_enabled helpers, JSONFormatter with exc_info, InMemoryImageCache._evict_oldest on empty store

Coverage improvement

Metric	Before	After
Total coverage	87%	95%
Missing lines	148	52
Tests	216	260

Key modules improved

• get_image.py: 83% → 100%
• config.py: 95% → 100%
• session.py: 71% → 100%
• observability/middleware.py: 0% → 100%
• observability/setup.py: 0% → 80%
• pillow_processor.py: 82% → 100%
• in_memory_cache.py: 98% → 100%
• logging_config.py: 93% → 100%
• health.py: 74% → 94%

Remaining uncovered (by design)

The remaining 52 lines are in OTel-gated branches in main.py (lifespan with OTel enabled, metrics endpoint) and DI factory wiring in dependencies.py — these require a live database or full OTel stack and are appropriately skipped in unit tests.

Version

2.2.2 → 2.2.3 (PATCH — tests only)

Checklist

• ruff check — all passed
• ruff format --check — all formatted
• mypy — no issues in 6 source files
• pytest — 260 passed
• CHANGELOG.md updated
• Version bumped in pyproject.toml