sys_prompt support in Agent (#938)

# What does this PR do?

The current default system prompt for llama3.2 tends to overindex on
tool calling and doesn't work well when the prompt does not require tool
calling.

This PR adds an option to override the default system prompt, and
organizes tool-related configs into a new config object.

- [ ] Addresses issue (#issue)


## Test Plan


LLAMA_STACK_CONFIG=together pytest
\-\-inference\-model=meta\-llama/Llama\-3\.3\-70B\-Instruct -s -v
tests/client-sdk/agents/test_agents.py::test_override_system_message_behavior


## Sources

Please link relevant resources if necessary.


## Before submitting

- [ ] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [ ] Ran pre-commit to handle lint / formatting issues.
- [ ] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

llama_stack/apis/agents/agents.py

@@ -33,6 +33,7 @@ from llama_stack.apis.inference import (
     ToolResponse,
     ToolResponseMessage,
     UserMessage,
+    ToolConfig,
 )
 from llama_stack.apis.safety import SafetyViolation
 from llama_stack.apis.tools import ToolDef
@@ -153,11 +154,24 @@ class AgentConfigCommon(BaseModel):
     output_shields: Optional[List[str]] = Field(default_factory=list)
     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=None)
+    tool_choice: Optional[ToolChoice] = Field(default=ToolChoice.auto, deprecated="use tool_config instead")
+    tool_prompt_format: Optional[ToolPromptFormat] = Field(default=None, deprecated="use tool_config instead")
+    tool_config: Optional[ToolConfig] = Field(default=None)
 
     max_infer_iters: Optional[int] = 10
 
+    def model_post_init(self, __context):
+        if self.tool_config:
+            if self.tool_choice and self.tool_config.tool_choice != self.tool_choice:
+                raise ValueError("tool_choice is deprecated. Use tool_choice in tool_config instead.")
+            if self.tool_prompt_format and self.tool_config.tool_prompt_format != self.tool_prompt_format:
+                raise ValueError("tool_prompt_format is deprecated. Use tool_prompt_format in tool_config instead.")
+        if self.tool_config is None:
+            self.tool_config = ToolConfig(
+                tool_choice=self.tool_choice,
+                tool_prompt_format=self.tool_prompt_format,
+            )
+
 
 @json_schema_type
 class AgentConfig(AgentConfigCommon):
@@ -268,6 +282,7 @@ class AgentTurnCreateRequest(AgentConfigOverridablePerTurn):
     toolgroups: Optional[List[AgentToolGroup]] = None
 
     stream: Optional[bool] = False
+    tool_config: Optional[ToolConfig] = None
 
 
 @json_schema_type
@@ -315,6 +330,7 @@ class Agents(Protocol):
         stream: Optional[bool] = False,
         documents: Optional[List[Document]] = None,
         toolgroups: Optional[List[AgentToolGroup]] = None,
+        tool_config: Optional[ToolConfig] = None,
     ) -> Union[Turn, AsyncIterator[AgentTurnResponseStreamChunk]]: ...
 
     @webmethod(route="/agents/{agent_id}/session/{session_id}/turn/{turn_id}", method="GET")