A race could occur when Close() called conn.Session.Close() concurrently
with an in-flight conn.Session.CallTool(), leading to undefined behavior.
Fix by adding a sync.WaitGroup to Manager:
- CallTool increments the WaitGroup while holding the read lock (after
checking m.closed), ensuring no new calls are counted after Close sets
the flag
- Close sets m.closed=true, releases the write lock, then waits for all
in-flight calls to finish via wg.Wait() before closing sessions
cfc29a1383