Close streams outside mutex in session cleanup

[koic]

Motivation and Context

cleanup_session_unsafe was closing streams inside the mutex, which could block all session operations if stream.close is slow or blocks (e.g., waiting for network I/O). The original reap_expired_sessions avoided this by closing streams outside the mutex, but the other 6 call sites of cleanup_session_unsafe did not, creating an inconsistency.

This change separates stream closing from session deletion. cleanup_session_unsafe now only removes the session from @sessions and returns it. All callers close streams outside the mutex via close_stream_safely, ensuring consistent behavior and preventing mutex contention from blocking I/O.

How Has This Been Tested?

Added tests that verify streams are closed outside the mutex by using Mutex#try_lock inside mock stream close callbacks:

• reap_expired_sessions closes stream outside mutex
• send_notification closes stream outside mutex on write error

Breaking Changes

None. This is an internal improvement with no change to public API behavior.

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