Ignore non-parseable lines on stdout.

[vorporeal]

Motivation and Context

As reported in #412, a non-conforming stdio transport-based server can break this client library by writing non-JSONRPC text to stdout before the initialization handshake.

This changes the JsonRpcMessageCodec::decode to ignore lines that don't parse as JSONRPC instead of propagating up an error.

How Has This Been Tested?

Tested this in Warp, using the @circleci/mcp-server-circleci server. We've had this change live in production (via a fork) for a week now and have not received any reports from users of it causing issues.

Breaking Changes

No, this does not affect any public APIs, and should strictly improve compatibility with stdio-based MCP servers.

Types of changes

• 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
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Important

I don't love this change. While it is effective (and was a good quick fix in our fork), it ultimately makes behavioral decisions in parsing code that belong in calling code.

I think the better fix here is to change the signature of Transport::receive() to produce Result<RxJsonRpcMessage<R>, _> instead of Option<RxJsonRpcMessage<R>>, so that we can surface errors to the service layer and let it make decisions. For example, the service layer might want to ignore decoding errors (and read another message), but handle I/O errors.

Figured I'd start by sending this change as-is in case y'all are ok with it as a quick fix for the problem, and want to make the API change later (given that it would be a semver breaking change).