fix: exclude LiteLlm llm_client from JSON serialization

The llm_client field holds a LiteLLMClient with no JSON representation, so
serializing a LiteLlm agent (as adk web and api_server do when returning
the agent graph) raised PydanticSerializationError and 500'd every request
for LiteLlm-based agents. Exclude the client from serialization; it is
runtime state, not data.

Close #6164

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 949034831

Author: GWeale

src/google/adk/models/lite_llm.py

@@ -2679,7 +2679,9 @@ class LiteLlm(BaseLlm):
     llm_client: The LLM client to use for the model.
   """
 
-  llm_client: LiteLLMClient = Field(default_factory=LiteLLMClient)
+  # LiteLLMClient has no JSON serializer, so it is excluded from dumps to keep
+  # model_dump(mode="json") from raising.
+  llm_client: LiteLLMClient = Field(default_factory=LiteLLMClient, exclude=True)
   """The LLM client to use for the model."""
 
   _additional_args: Dict[str, Any] = None

tests/unittests/models/test_litellm.py

@@ -6335,3 +6335,14 @@ async def test_streaming_tool_call_brace_in_string_does_not_falsely_complete(
   args_by_name = {p.function_call.name: p.function_call.args for p in parts}
   assert args_by_name["my_func"] == json.loads(full_args_a)
   assert args_by_name["other_func"] == json.loads(full_args_b)
+
+
+def test_model_dump_json_excludes_llm_client():
+  lite_llm_model = LiteLlm(model="test_model")
+
+  dumped = lite_llm_model.model_dump(mode="json")
+  dumped_json = lite_llm_model.model_dump_json()
+
+  assert "llm_client" not in dumped
+  assert "llm_client" not in json.loads(dumped_json)
+  assert dumped["model"] == "test_model"