Improve CLI path verification in CopilotClient to use shutil.which

Author: brettcannon

python/copilot/client.py

@@ -16,6 +16,7 @@
 import inspect
 import os
 import re
+import shutil
 import subprocess
 import sys
 import threading
@@ -1268,7 +1269,8 @@ async def _start_cli_server(self) -> None:
 
         # Verify CLI exists
         if not os.path.exists(cli_path):
-            raise RuntimeError(f"Copilot CLI not found at {cli_path}")
+            if (cli_path := shutil.which(cli_path)) is None:
+                raise RuntimeError(f"Copilot CLI not found at {cli_path}")
 
         # Start with user-provided cli_args, then add SDK-managed args
         args = list(cfg.cli_args) + [