#2766 fix(server): return stdio parse errors

Summary

• make the stdio server return JSON-RPC error frames when incoming lines fail JSON-RPC parsing/validation
• recover a simple request id from the raw line when possible so id-bearing malformed requests do not stay pending
• keep later valid stdio messages flowing after an invalid line

Fixes #2751.

To verify

• uv run pytest tests/server/test_stdio.py -q
• uv run ruff check src/mcp/server/stdio.py tests/server/test_stdio.py
• uv run pyright src/mcp/server/stdio.py tests/server/test_stdio.py
• git diff --check