style: ruff format fix

Author: Sarvesh

pr_body.md

@@ -0,0 +1,15 @@
+## Summary
+- Changes the default `encoding_error_handler` in `StdioServerParameters` from `"strict"` to `"replace"`.
+- Malformed UTF-8 bytes from a child server stdout previously crashed the client transport with `UnicodeDecodeError`.
+- With `"replace"`, invalid bytes become U+FFFD and the line fails JSON-RPC validation, which is surfaced as an in-stream `Exception` that the session can handle.
+- This mirrors the server-side stdio hardening from PR #2302.
+
+## Test Plan
+- [x] Reproduced the crash with a child process emitting `\xff\xfe\n` followed by valid JSON-RPC.
+- [x] Verified the fix: first item is a `ValidationError`, second item is the valid `SessionMessage`.
+- [x] Added regression test `test_invalid_utf8_from_the_server_surfaces_as_an_in_stream_exception`.
+- [x] Full `tests/client/test_stdio.py` suite passes (34 passed, 1 skipped).
+
+## Notes
+- Fixes #2454
+- Scope intentionally small.

src/mcp/server/mcpserver/server.py

@@ -389,9 +389,7 @@ async def call_tool(
         if isinstance(result, CallToolResult):
             return result
         if isinstance(result, tuple) and len(result) == 2:  # type: ignore[arg-type]
-            unstructured_content, structured_content = cast(
-                tuple[Sequence[ContentBlock], dict[str, Any]], result
-            )
+            unstructured_content, structured_content = cast(tuple[Sequence[ContentBlock], dict[str, Any]], result)
             return CallToolResult(
                 content=list(unstructured_content),
                 structured_content=structured_content,