Generic 500 body in outer POST except; make leak-test assertion load-bearing

The outer except now returns a fixed message instead of embedding the
exception text in the response body, matching the convention in runner.py
and jsonrpc_dispatcher.py (detail goes to logger.exception).

Moves the _request_streams/_sse_stream_writers emptiness assertions inside
the connect() context so connect()'s own teardown doesn't make them vacuous,
and asserts the store's exception message is absent from the 500 body.

Author: maxisbey

src/mcp/server/streamable_http.py

@@ -641,7 +641,7 @@ async def _handle_post_request(self, scope: Scope, request: Request, receive: Re
         except Exception as err:
             logger.exception("Error handling POST request")
             response = self._create_error_response(
-                f"Error handling POST request: {err}",
+                "Error handling POST request",
                 HTTPStatus.INTERNAL_SERVER_ERROR,
                 INTERNAL_ERROR,
             )

tests/server/test_streamable_http_router.py

@@ -151,8 +151,12 @@ async def asgi_send(message: Message) -> None:
             with anyio.fail_after(5):
                 forwarded = await read_stream.receive()
             assert isinstance(forwarded, Exception)
+        # handle_request has returned; connect()'s finally (which clears
+        # _request_streams unconditionally) has not yet run.
+        assert transport._request_streams == {}
+        assert transport._sse_stream_writers == {}
 
-    assert transport._request_streams == {}
-    assert transport._sse_stream_writers == {}
     assert sent[0]["type"] == "http.response.start"
     assert sent[0]["status"] == 500
+    body = b"".join(m.get("body", b"") for m in sent if m["type"] == "http.response.body")
+    assert b"backend unavailable" not in body