Add a closed flag to the Manager struct to prevent CallTool from
accessing server connections after Close has been called. The flag
is checked within the RLock in CallTool to ensure thread-safety.
Previously, CallTool could obtain a server reference using RLock,
then that reference could be closed by Close() running concurrently,
leading to use-after-close errors. Now:
1. CallTool checks the closed flag before accessing servers
2. Close sets the closed flag before closing connections
3. CallTool directly accesses m.servers within the lock instead
of using GetServer() to avoid releasing the lock prematurely
This ensures CallTool will not use a server connection that is
being closed or has been closed.
Addresses Copilot code review feedback.
f1b798434d