…chafik/pr1222

…eamablehttp_client

- Add StreamableHTTPClient class with better encapsulation
- Support httpx_client_factory and auth parameters
- Maintain backwards compatibility with async context manager interface
- Provide connect() method for explicit connection management
- Add comprehensive docstrings and type annotations

The new class provides the same 3-tuple interface (read_stream, write_stream, get_session_id)
as the original streamablehttp_client function, ensuring full backwards compatibility.

…ableHTTPClient

- Replace streamablehttp_client implementation to use StreamableHTTPClient internally
- Add deprecation warning to guide users to the new class
- Maintain full backwards compatibility
- All tests pass with the new implementation

The function now delegates to the new StreamableHTTPClient class, ensuring
consistent behavior while encouraging migration to the cleaner class-based API.