Summary

Fixes a streamable HTTP client hang when a POST request receives an SSE response that closes before sending a JSON-RPC response.

This targets #1577. It overlaps with the older #1578, but keeps the current-main patch narrow: preserve the existing reconnect behavior when a resumable event id is available, and only resolve the pending request with the existing SDK CONNECTION_CLOSED error when there is no response and no resumable event to follow.

Root Cause

_handle_sse_response() consumed the POST SSE stream and returned silently if the stream ended before a response and no Last-Event-ID had been observed. Because no JSONRPCError or transport exception was written to the read stream, the JSON-RPC dispatcher kept the corresponding call_tool() waiter pending forever.

Changes

• Send a correlated JSONRPCError(CONNECTION_CLOSED) when a POST SSE response closes before a reply and cannot be resumed.
• Also resolve the pending request with CONNECTION_CLOSED if reconnection attempts are exhausted.
• Add a non-SDK server regression test that closes an SSE response without a JSON-RPC reply and asserts call_tool() raises instead of hanging.

Validation

• uv run --frozen pytest tests/client/test_notification_response.py::test_empty_post_sse_response_unblocks_pending_tool_call -q
• uv run --frozen pytest tests/client/test_notification_response.py -q
• uv run --frozen pytest tests/shared/test_streamable_http.py -q
• uv run --frozen ruff format src/mcp/client/streamable_http.py tests/client/test_notification_response.py --check
• uv run --frozen ruff check src/mcp/client/streamable_http.py tests/client/test_notification_response.py
• uv run --frozen pyright src/mcp/client/streamable_http.py tests/client/test_notification_response.py
• uv run --frozen coverage erase && uv run --frozen coverage run -m pytest tests/client/test_notification_response.py tests/shared/test_streamable_http.py && uv run --frozen coverage combine && uv run --frozen coverage report --include='src/mcp/client/streamable_http.py' --fail-under=0 && UV_FROZEN=1 uv run --frozen strict-no-cover