When a model returns an empty response after tool calls with no new
tool_calls in the follow-up turn, the code enters the "nudge" recovery
path which referenced `assistant_msg` before it was assigned. This
variable is only set in the tool-calls branch (line 10098), but the
nudge code lives in the no-tool-calls branch (line 10263+).
The fix builds a fresh assistant message dict via `_build_assistant_message()`
instead of reusing the unbound variable, consistent with the exhausted-
retries path at line 10457.
