mcp(fix[pane_tools]): Use pane.kill() instead of raw cmd

why: Raw pane.cmd("kill-pane") skips stderr checking and structured
logging that Pane.kill() provides.
what:
- Replace pane.cmd("kill-pane") with pane.kill()

Author: tony

src/libtmux/mcp/tools/pane_tools.py

@@ -203,7 +203,7 @@ def kill_pane(
         window_id=window_id,
     )
     pid = pane.pane_id
-    pane.cmd("kill-pane")
+    pane.kill()
     return f"Pane killed: {pid}"