Keep stdio_client strict-no-cover in sync with exercised behavior

The malformed-UTF8 regression test now executes the JSON-parse failure branch in stdio_client, so the old pragma no longer reflects reality and breaks CI's strict-no-cover gate. This follow-up removes the stale pragma without changing behavior.

Constraint: Must not widen the PR scope beyond the already-proposed stdio robustness fix
Rejected: Suppress strict-no-cover or weaken the regression test | hides real coverage drift instead of correcting it
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: When adding malformed-input regression tests, re-audit nearby no-cover pragmas immediately
Tested: coverage run of tests/client/test_stdio.py plus strict-no-cover locally up to the updated branch execution path
Not-tested: Full upstream CI rerun after push

Author: shaun0927

src/mcp/client/stdio.py

@@ -151,7 +151,7 @@ async def stdout_reader():
                     for line in lines:
                         try:
                             message = types.jsonrpc_message_adapter.validate_json(line, by_name=False)
-                        except Exception as exc:  # pragma: no cover
+                        except Exception as exc:
                             logger.exception("Failed to parse JSONRPC message from server")
                             await read_stream_writer.send(exc)
                             continue