fix: get_llm_models handler returns UUID strings instead of full model dicts (#2081)

The plugin SDK declares get_llm_models() -> list[str] (UUID strings),
but the host handler returned the full model dict list from
llm_model_service.get_llm_models(). This caused TypeError when
invoke_llm passed a dict to get_model_by_uuid (which is decorated
with @async_lru and requires hashable arguments).

Extract only the 'uuid' field to match the SDK contract.

src/langbot/pkg/plugin/handler.py

@@ -314,11 +314,11 @@ class RuntimeConnectionHandler(handler.Handler):
 
         @self.action(PluginToRuntimeAction.GET_LLM_MODELS)
         async def get_llm_models(data: dict[str, Any]) -> handler.ActionResponse:
-            """Get llm models"""
+            """Get llm models, returns list of UUID strings"""
             llm_models = await self.ap.llm_model_service.get_llm_models(include_secret=False)
             return handler.ActionResponse.success(
                 data={
-                    'llm_models': llm_models,
+                    'llm_models': [m['uuid'] for m in llm_models],
                 },
             )