fix: drain terminal streamable HTTP responses

[he-yufeng]

Summary

Fixes #2707.

The streamable HTTP client currently closes SSE responses as soon as it reads the terminal JSON-RPC response or error. With HTTP keepalive, that can leave unread response bytes on the connection and force the next request down a fresh connection path.

This changes the request SSE paths to treat the terminal JSON-RPC message as logical completion, but still drain the response stream to EOF before returning. Cancellation and shutdown paths still close the stream via the existing context managers.

Testing

uv run pytest tests/shared/test_streamable_http.py -k "drains_after_terminal_event" -q
uv run pytest tests/shared/test_streamable_http.py -q
uv run ruff check src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py
uv run ruff format --check src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py
git diff --check

[he-yufeng]

Closing this because #2712 already covers the same streamable HTTP SSE drain path. I missed that existing PR during duplicate search, so keeping this open would only add review noise.