make default tool prompt format none in agent config (#863)

# What does this PR do?

Previously the tests hard coded the tool prompt format to be json which
will cause it to fail when using 3.2/3.3 family of models. This change
make the default to be none for the agent config and just remove the
specification in the tests.


## Test Plan
LLAMA_STACK_BASE_URL=http://localhost:8321 pytest -v
tests/client-sdk/agents/test_agents.py

llama_stack/apis/agents/agents.py

@@ -155,9 +155,7 @@ class AgentConfigCommon(BaseModel):
     toolgroups: Optional[List[AgentToolGroup]] = Field(default_factory=list)
     client_tools: Optional[List[ToolDef]] = Field(default_factory=list)
     tool_choice: Optional[ToolChoice] = Field(default=ToolChoice.auto)
-    tool_prompt_format: Optional[ToolPromptFormat] = Field(
-        default=ToolPromptFormat.json
-    )
+    tool_prompt_format: Optional[ToolPromptFormat] = Field(default=None)
 
     max_infer_iters: int = 10
 

tests/client-sdk/agents/test_agents.py

@@ -98,7 +98,6 @@ def agent_config(llama_stack_client, text_model_id):
         },
         toolgroups=[],
         tool_choice="auto",
-        tool_prompt_format="json",
         input_shields=available_shields,
         output_shields=available_shields,
         enable_session_persistence=False,