revert: undo session.py pragma change (speculative, bad evidence)

The session.py:426 flag appeared exactly once across 10 runs, and that
one run was the only one with a confounding test edit in place. CI has
never flagged this line; 8 clean local runs never flagged it either.
The original pragma was correct.

Author: maxisbey

src/mcp/shared/session.py

@@ -423,7 +423,7 @@ async def _receive_loop(self) -> None:
                     try:
                         await stream.send(JSONRPCError(jsonrpc="2.0", id=id, error=error))
                         await stream.aclose()
-                    except Exception:  # pragma: lax no cover
+                    except Exception:  # pragma: no cover
                         # Stream might already be closed
                         pass
                 self._response_streams.clear()