Fix RunEngine thread blocking on engine.play() after kill signal

When the analysis thread is interrupted via _kill.set(), if no bestmove
was found yet (bm is None), RunEngine.run() falls through to
engine.play(board, Limit(depth=None)) — an unconstrained synchronous
call that blocks the thread indefinitely. This makes join() hang and
freezes the GUI.

Fix: skip the engine.play() fallback when _kill is set, and also make
the move delay loop respect the kill signal.

Agent-Logs-Url: https://github.com/fsmosca/Python-Easy-Chess-GUI/sessions/225e88fb-0a7d-479d-8208-22d1eb8867e0

Co-authored-by: fsmosca <22366935+fsmosca@users.noreply.github.com>

Author: Copilot

python_easy_chess_gui.py

@@ -691,13 +691,17 @@ def run(self):
         # Apply engine move delay if movetime is small
         if self.is_move_delay:
             while True:
-                if time.perf_counter() - start_time >= self.move_delay_sec:
+                if (self._kill.is_set()
+                        or time.perf_counter() - start_time
+                        >= self.move_delay_sec):
                     break
                 logging.info('Delay sending of best move {}'.format(self.bm))
                 time.sleep(1.0)
 
         # If bm is None, we will use engine.play()
-        if self.bm is None:
+        # Skip this fallback when the search was explicitly interrupted
+        # to avoid blocking the thread with an unconstrained engine call.
+        if self.bm is None and not self._kill.is_set():
             logging.info('bm is none, we will try engine,play().')
             try:
                 result = self.engine.play(self.board, limit)