fix(github_copilot): main.py header block honors resolved api_key

The openai-compatible completion branch in main.py builds the Copilot
default headers with a fresh Authenticator().get_api_key() regardless
of the api_key already resolved for the call, so a clientside per-user
key raised GetAPIKeyError (mapped to AuthenticationError) whenever the
file-backed authenticator was unseeded. Use the resolved api_key and
only fall back to the authenticator when none is present. Found in
end-to-end verification of per-user Copilot credentials.

Author: tanmaykm

litellm/main.py

@@ -2381,8 +2381,13 @@ def _complete_custom_openai(
             get_copilot_default_headers,
         )
 
-        copilot_auth = Authenticator()
-        copilot_api_key = copilot_auth.get_api_key()
+        # a caller-supplied (clientside / per-user) key was already resolved
+        # into api_key above; only fall back to the global file-backed
+        # authenticator when no key is present
+        copilot_api_key = api_key
+        if copilot_api_key is None:
+            copilot_auth = Authenticator()
+            copilot_api_key = copilot_auth.get_api_key()
         copilot_headers = get_copilot_default_headers(copilot_api_key)
         if extra_headers:
             copilot_headers.update(extra_headers)