Fix SSE Transport Memory Leak

[lnbc1QWFyb24]

When running the SSE transport in production, we noticed a massive memory leak. We have a Kubernetes health check process that was hitting the SSE endpoint periodically and then just disconnecting which was causing the memory leak. Digging in to the current implementation, there is no disconnection handling in the case of an abrupt disconnection, so this PR adds some cleanup when the connection drops.

Motivation and Context

Removes a memory leak in the case that a client disconnects abruptly.

How Has This Been Tested?

You can replicate the memory leak by simply making a GET request to the SSE endpoint and then disconnecting many times to see memory grow quite rapidly. I applied this fix to our production environment and the instance memory usage has been steady since.

Breaking Changes

No

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

There were two errors that were failing on main already, so these changes have not added any new errors.

[4t145]

Good point, would it better to release the resource when SseServerTransport drop?

impl Drop for SseServerTransport {
    // release the resource...
}

[lnbc1QWFyb24]

In the case of a disconnection, the SseServerTransport is never dropped since it is not cleared from the tx store upon disconnection. When it is cleared from the tx store it is dropped though.

[4t145]

LGTM