fix(cli): Serialize LiteLlm graph models safely

The graph serializer currently passes model fields through when their value is not a simple collection or nested agent. For LiteLlm that leaves the runtime LiteLLMClient object inside the serialized graph payload, so JSON encoding the graph fails.

This patch serializes BaseLlm values using their model name string value.model to ensure JSON safety and match the web UI frontend's expected string schema.

Merge #5956
closes #5949

Change-Id: Ie31f640f6d9bedd515fd85ad0d4de2f5ad1ef7d7

Author: he-yufeng

src/google/adk/cli/utils/graph_serialization.py

@@ -22,6 +22,7 @@
 logger = logging.getLogger("google_adk." + __name__)
 
 from ...agents.base_agent import BaseAgent
+from ...models.base_llm import BaseLlm
 from ...tools.base_toolset import BaseToolset
 
 # Node type mapping for cleaner lookup
@@ -227,6 +228,8 @@ def serialize_agent(agent: BaseAgent) -> dict[str, Any]:
         # Handle nested agents
         if isinstance(value, BaseAgent):
           agent_dict[field_name] = serialize_agent(value)
+        elif isinstance(value, BaseLlm):
+          agent_dict[field_name] = value.model
         # Handle simple types and collections
         elif isinstance(value, (str, int, float, bool, list, dict)):
           agent_dict[field_name] = value

tests/unittests/cli/utils/test_graph_serialization.py

@@ -14,7 +14,11 @@
 
 """Tests for graph_serialization edge handling with routing maps."""
 
+import json
+
+from google.adk.agents import LlmAgent
 from google.adk.cli.utils.graph_serialization import serialize_agent
+from google.adk.models.lite_llm import LiteLlm
 from google.adk.tools.base_toolset import BaseToolset
 from google.adk.workflow import START
 from google.adk.workflow import Workflow
@@ -126,3 +130,15 @@ def __init__(self):
   assert len(result['tools']) == 1
   assert result['tools'][0]['name'] == 'MockToolset'
   assert result['tools'][0]['type'] == 'tool'
+
+
+def test_serialize_agent_with_litellm_model_is_json_safe() -> None:
+  agent = LlmAgent(
+      name='repro',
+      model=LiteLlm(model='ollama_chat/llama3'),
+  )
+
+  result = serialize_agent(agent)
+
+  assert result['model'] == 'ollama_chat/llama3'
+  json.dumps(result)