Add error log for client stdio

[pengwa]

When use stdio mode, the server module is provided by developers (like me). But sometimes, the module not only call FastMCP().run(), but also other code. If the other code print something, then standard output will contains those text.

When stdio client read the standard output of the server module, it will fail to parse the content in expected content.

For example:

[50] Add mon5_log_handler to original_logger fsspec.mapping{"jsonrpc":"2.0","id":0,"result":{"protocolVersion":"2025-03-26","capabilities":{"experimental":{},"prompts":{"listChanged":false},"resources":{"subscribe":false,"listChanged":false},"tools":{"listChanged":false}},"serverInfo":{"name":"draft_2025-06-10 13:37:18","version":"0.1.0"}}} """

[50] Add mon5_log_handler to original_logger is the log I printed by mistake before calling FastMCP().run().

Motivation and Context

It is really hard to finally locate the root cause, so we need a better logging showing such error clearly.

How Has This Been Tested?

Tested in a stdio mode. Let me know if you have any suggestions on what else test to be covered.

Breaking Changes

NO!

Types of changes

• [ x] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• [x ] New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[pengwa]

Resolved format issue. Please take a further look. @Kludex. THanks!

[felixweinberger]

Hi @pengwa thanks for this contribution! And apologies for the time it took to get back to you on this.

Change looks good to me, let's just fix the duplicate logger instantiation - happy to get back to this quickly if you have bandwidth to make the update?

[pengwa]

Hi @pengwa thanks for this contribution! And apologies for the time it took to get back to you on this.

Change looks good to me, let's just fix the duplicate logger instantiation - happy to get back to this quickly if you have bandwidth to make the update?

Thanks @felixweinberger , thanks for reminding me there is such a PR here. Just updated the code, feel free to review again.

[felixweinberger]

@pengwa thanks for updating your PR.

Dismissing review as it's been addressed.