fix: compare bound method by equality, not identity

Bound methods are created fresh on each attribute access, so
'self._missing_function_call_handler is self._missing_function_call_handler'
is always False. Using 'is' meant the placeholder branch never fired and
both warnings logged when a function was missing at queue time.

Switch to == so equality compares the underlying function and instance.
Strengthen the missing-at-queue-time test to assert the second warning
does not fire.

src/pipecat/services/llm_service.py

@@ -792,7 +792,7 @@ class LLMService(UserTurnCompletionLLMServiceMixin, AIService):
             item = self._functions[runner_item.function_name]
         elif None in self._functions.keys():
             item = self._functions[None]
-        elif runner_item.registry_item.handler is self._missing_function_call_handler:
+        elif runner_item.registry_item.handler == self._missing_function_call_handler:
             item = runner_item.registry_item
         else:
             logger.warning(