test(server): cover terminate cleanup of orphaned request streams

Exercise terminate()'s remaining _request_streams loop so CI
fail-under=100% coverage passes for streamable_http (#2150 / PR #3125).
Also mark the defensive registered_stream_id guard as no-branch.

Author: AgentSmithClaw

src/mcp/server/streamable_http.py

@@ -934,7 +934,9 @@ async def send_event(event_message: EventMessage) -> None:
 
                                         await sse_stream_writer.send(event_data)
                             finally:
-                                if registered_stream_id is not None:
+                                # registered_stream_id is set immediately on try entry; keep the
+                                # guard for defensive cleanup if re-key is later reordered.
+                                if registered_stream_id is not None:  # pragma: no branch
                                     self._sse_stream_writers.pop(registered_stream_id, None)
                                 await self._clean_up_memory_streams(stream_id)
                 except anyio.ClosedResourceError:  # pragma: lax no cover

tests/server/test_streamable_http_manager.py

@@ -758,11 +758,16 @@ async def test_terminate_closes_active_sse_stream_writers():
     transport = StreamableHTTPServerTransport(mcp_session_id="test-session-2150")
     send_stream, receive_stream = anyio.create_memory_object_stream[object](1)
     transport._sse_stream_writers["req-1"] = send_stream  # type: ignore[assignment]
+    # Orphaned request stream with a key NOT also in _sse_stream_writers: close_sse_stream
+    # only pops matching request streams, so terminate's remaining-keys loop must run.
+    req_send, req_recv = anyio.create_memory_object_stream[object](1)
+    transport._request_streams["req-orphan"] = (req_send, req_recv)  # type: ignore[assignment]
 
     await transport.terminate()
 
     assert transport.is_terminated
     assert "req-1" not in transport._sse_stream_writers
+    assert not transport._request_streams
     with pytest.raises(anyio.ClosedResourceError):
         await send_stream.send(object())  # type: ignore[arg-type]
     await receive_stream.aclose()