fix: drain terminal streamable HTTP responses

[he-yufeng]

Summary

• stop closing terminal Streamable HTTP SSE responses immediately after the matching JSON-RPC response
• keep draining the POST, GET resumption, and standalone GET SSE streams to EOF so HTTP keepalive connections stay reusable
• add regression coverage for both normal response handling and reconnection after a terminal event

Fixes #2707.

To verify

• 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 pyright src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py
• git diff --check

[he-yufeng]

Rebased this branch on current main and force-pushed 93a88fa.

The conflict was in tests/shared/test_streamable_http.py because upstream added newer priming-event coverage around the same area. I kept the upstream test shape and re-applied this PR's drain-after-terminal-event coverage before it.

Validated locally:

• uv run --frozen pytest tests/shared/test_streamable_http.py::test_handle_sse_response_drains_after_terminal_event tests/shared/test_streamable_http.py::test_reconnection_drains_after_terminal_event tests/shared/test_streamable_http.py::test_streamable_http_client_session_termination tests/shared/test_streamable_http.py::test_streamable_http_client_session_termination_204 tests/interaction/transports/test_hosting_resume.py -q (10 passed)
• uv run --frozen ruff check src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py tests/interaction/transports/test_hosting_resume.py
• uv run --frozen ruff format --check src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py tests/interaction/transports/test_hosting_resume.py
• uv run --frozen pyright src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py tests/interaction/transports/test_hosting_resume.py (0 errors)
• python -m py_compile src\mcp\client\streamable_http.py tests\shared\test_streamable_http.py tests\interaction\transports\test_hosting_resume.py
• git diff --check --cached before completing the rebase