fix: agent config validation (#1053)

Summary:

Fixes AgentConfig init bug introduced with ToolConfig.

Namely, the below doesn't work
```
    agent_config = AgentConfig(
        **common_params,
        tool_config=ToolConfig(
            tool_choice="required",
        ),
    )
```
bvecause tool_choice was defaulted to 'auto' leading to validation check
failing.

Test Plan:

added unittests

LLAMA_STACK_CONFIG=fireworks pytest -s -v tests/client-sdk/
--safety-shield meta-llama/Llama-Guard-3-8B

llama_stack/providers/inline/agents/meta_reference/agents.py

@@ -81,12 +81,6 @@ class MetaReferenceAgentsImpl(Agents):
     ) -> AgentCreateResponse:
         agent_id = str(uuid.uuid4())
 
-        if agent_config.tool_config is None:
-            agent_config.tool_config = ToolConfig(
-                tool_choice=agent_config.tool_choice,
-                tool_prompt_format=agent_config.tool_prompt_format,
-            )
-
         await self.persistence_store.set(
             key=f"agent:{agent_id}",
             value=agent_config.model_dump_json(),