fix: Properly track session.json debounce timer for cleanup

- Update session.sessionJsonDebounceTimer when creating new timers
- Clear timer reference after callback execution
- Remove incorrect initial null assignment
- Fixes memory leak where timers continued after session cleanup

The bug occurred because the timer ID was only stored in a local variable,
preventing proper cleanup during session termination.

web/src/server/pty/pty-manager.ts

@@ -818,7 +818,7 @@ export class PtyManager extends EventEmitter {
             // Clear the timer reference after execution
             session.sessionJsonDebounceTimer = null;
           }, 100);
-          
+
           // Update the session's timer reference
           session.sessionJsonDebounceTimer = debounceTimer;
         }