fix: cast Anthropic-format passthrough message to MessageParam

The fallback path in `_from_universal_context_message` returns
`message.message` from an `LLMSpecificMessage`, which is typed loosely
(`Any | dict`). The surrounding comment already documents the
assumption that the message is already in Anthropic format — make that
assumption explicit to pyright with a cast.

src/pipecat/adapters/services/anthropic_adapter.py

@@ -248,7 +248,7 @@ class AnthropicLLMAdapter(BaseLLMAdapter[AnthropicLLMInvocationParams]):
             }
 
         # Fall back to assuming that the message is already in Anthropic format
-        return copy.deepcopy(message.message)
+        return cast(MessageParam, copy.deepcopy(message.message))
 
     def _from_standard_message(self, message: LLMStandardMessage) -> MessageParam:
         """Convert standard universal context message to Anthropic format.