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.
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