fix: guard agent field serializer against None when profile-only request is dumped

`StartConversationRequest` with `agent_profile_id` set holds `agent=None`
(the server resolves it). Pydantic's `model_dump` calls the `AgentBase`
model_serializer with `self=None`, crashing. The `@field_serializer("agent",
mode="wrap")` short-circuits before the model_serializer fires and returns
`None` directly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Simon Rosenberg

openhands-sdk/openhands/sdk/conversation/request.py

@@ -11,7 +11,14 @@
 from typing import Annotated, Any, Literal, cast
 from uuid import UUID
 
-from pydantic import BaseModel, Discriminator, Field, Tag, model_validator
+from pydantic import (
+    BaseModel,
+    Discriminator,
+    Field,
+    Tag,
+    field_serializer,
+    model_validator,
+)
 
 from openhands.sdk.agent.acp_agent import ACPAgent as ACPAgent
 from openhands.sdk.agent.agent import Agent as Agent
@@ -287,6 +294,12 @@ def _require_agent(self) -> StartConversationRequest:
             )
         return self
 
+    @field_serializer("agent", mode="wrap")
+    def _serialize_agent(self, value: AgentBase | None, handler: Any) -> Any:
+        if value is None:
+            return None
+        return handler(value)
+
 
 class StartACPConversationRequest(StartConversationRequest):
     """Deprecated compatibility alias for ACP-capable start requests.