Add HttpClientManager class for managing httpx.AsyncClient pools with
FastAPI lifespan integration. Features include:
- Default shared connection pool for general requests
- Configurable max connections, keep-alive, and timeouts
- Dedicated thread pool for blocking I/O operations
- Graceful startup/shutdown lifecycle management
- Per-upstream client isolation support (for future use)
Includes comprehensive unit tests covering initialization, startup,
shutdown, client retrieval, blocking operations, idempotency, and
error handling.
- Remove Tag/TagHistory model tests from unit tests
- Update CacheSettings tests to remove allow_public_internet field
- Replace tag= with version= in upload_test_file calls
- Update test assertions to use versions instead of tags
- Remove tests for tag: prefix downloads (now uses version:)
- Update dependency tests for version-only schema
