fix(copilot): guard request_flags against a non-dict last message (#5274)

request_flags derives (agent, vision) and does last.get("role") after only
a truthy check. A client can send a bare-string message element
("messages": ["hi"]), and the vision loop right below already guards each
element with isinstance — so the .get() on a non-dict last element is an
oversight that raises AttributeError on every Copilot-proxied request with
such a body.

Use isinstance(last, dict) to match the loop's own guard.

Fixes #5273

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

src/copilot.py

@@ -230,7 +230,10 @@ def request_flags(messages) -> tuple:
     """
     msgs = messages or []
     last = msgs[-1] if msgs else None
-    agent = bool(last) and last.get("role") != "user"
+    # A message element can be a non-dict (clients send `"messages": ["hi"]`);
+    # the vision loop below already guards each element with isinstance, so do
+    # the same here rather than call .get() on a bare string.
+    agent = isinstance(last, dict) and last.get("role") != "user"
     vision = False
     for m in msgs:
         content = m.get("content") if isinstance(m, dict) else None