Fix OpenAI response input for async-openai 0.34

async-openai 0.34 adds a required phase field to EasyInputMessage,
which broke our Responses API request construction.

Set phase to None because baibot does not currently model assistant commentary vs final-answer turns,
so omitting phase preserves the previous behavior while keeping the request compatible with the new crate.

src/agent/provider/openai/utils.rs

@@ -56,6 +56,7 @@ pub fn convert_llm_messages_to_openai_response_input(
             r#type: MessageType::Message,
             role,
             content,
+            phase: None,
         }));
     }