feat(schema): add optional name field to Message model

Author: jinliyl

flowllm/core/schema/message.py

@@ -61,6 +61,7 @@ def simple_dump(self) -> dict:
 class Message(BaseModel):
     """Represents a message in a conversation with role, content, and optional tool calls."""
 
+    name: str | None = Field(default=None)
     role: Role = Field(default=Role.USER)
     content: str | List[ContentBlock] = Field(default="")
     reasoning_content: str = Field(default="")