Support sys_prompt behavior in inference (#937)

# 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

python -m unittest
llama_stack.providers.tests.inference.test_prompt_adapter


## 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.
---
[//]: # (BEGIN SAPLING FOOTER)
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[ReviewStack](https://reviewstack.dev/meta-llama/llama-stack/pull/937).
* #938
* __->__ #937

llama_stack/apis/inference/inference.py

@@ -308,14 +308,49 @@ class CompletionResponseStreamChunk(BaseModel):
     logprobs: Optional[List[TokenLogProbs]] = None
 
 
+class SystemMessageBehavior(Enum):
+    """Config for how to override the default system prompt.
+
+    :cvar append: Appends the provided system message to the default system prompt:
+        https://www.llama.com/docs/model-cards-and-prompt-formats/llama3_2/#-function-definitions-in-the-system-prompt-
+    :cvar replace: Replaces the default system prompt with the provided system message. The system message can include the string
+        '{{function_definitions}}' to indicate where the function definitions should be inserted.
+    """
+
+    append = "append"
+    replace = "replace"
+
+
+@json_schema_type
+class ToolConfig(BaseModel):
+    """Configuration for tool use.
+
+    :param tool_choice: (Optional) Whether tool use is required or automatic. Defaults to ToolChoice.auto.
+    :param tool_prompt_format: (Optional) Instructs the model how to format tool calls. By default, Llama Stack will attempt to use a format that is best adapted to the model.
+        - `ToolPromptFormat.json`: The tool calls are formatted as a JSON object.
+        - `ToolPromptFormat.function_tag`: The tool calls are enclosed in a <function=function_name> tag.
+        - `ToolPromptFormat.python_list`: The tool calls are output as Python syntax -- a list of function calls.
+    :param system_message_behavior: (Optional) Config for how to override the default system prompt.
+        - `SystemMessageBehavior.append`: Appends the provided system message to the default system prompt.
+        - `SystemMessageBehavior.replace`: Replaces the default system prompt with the provided system message. The system message can include the string
+            '{{function_definitions}}' to indicate where the function definitions should be inserted.
+    """
+
+    tool_choice: Optional[ToolChoice] = Field(default=ToolChoice.auto)
+    tool_prompt_format: Optional[ToolPromptFormat] = Field(default=None)
+    system_message_behavior: SystemMessageBehavior = Field(default=SystemMessageBehavior.append)
+
+
 # This is an internally used class
+@json_schema_type
 class ChatCompletionRequest(BaseModel):
     model: str
     messages: List[Message]
     sampling_params: Optional[SamplingParams] = SamplingParams()
+
     tools: Optional[List[ToolDefinition]] = Field(default_factory=list)
-    tool_choice: Optional[ToolChoice] = Field(default=ToolChoice.auto)
-    tool_prompt_format: Optional[ToolPromptFormat] = Field(default=None)
+    tool_config: Optional[ToolConfig] = Field(default_factory=ToolConfig)
+
     response_format: Optional[ResponseFormat] = None
     stream: Optional[bool] = False
     logprobs: Optional[LogProbConfig] = None
@@ -404,6 +439,7 @@ class Inference(Protocol):
         response_format: Optional[ResponseFormat] = None,
         stream: Optional[bool] = False,
         logprobs: Optional[LogProbConfig] = None,
+        tool_config: Optional[ToolConfig] = None,
     ) -> Union[ChatCompletionResponse, AsyncIterator[ChatCompletionResponseStreamChunk]]:
         """Generate a chat completion for the given messages using the specified model.
 
@@ -412,15 +448,20 @@ class Inference(Protocol):
         :param sampling_params: Parameters to control the sampling strategy
         :param tools: (Optional) List of tool definitions available to the model
         :param tool_choice: (Optional) Whether tool use is required or automatic. Defaults to ToolChoice.auto.
+            .. deprecated::
+               Use tool_config instead.
         :param tool_prompt_format: (Optional) Instructs the model how to format tool calls. By default, Llama Stack will attempt to use a format that is best adapted to the model.
             - `ToolPromptFormat.json`: The tool calls are formatted as a JSON object.
             - `ToolPromptFormat.function_tag`: The tool calls are enclosed in a <function=function_name> tag.
             - `ToolPromptFormat.python_list`: The tool calls are output as Python syntax -- a list of function calls.
+            .. deprecated::
+               Use tool_config instead.
         :param response_format: (Optional) Grammar specification for guided (structured) decoding. There are two options:
             - `ResponseFormat.json_schema`: The grammar is a JSON schema. Most providers support this format.
             - `ResponseFormat.grammar`: The grammar is a BNF grammar. This format is more flexible, but not all providers support it.
         :param stream: (Optional) If True, generate an SSE event stream of the response. Defaults to False.
         :param logprobs: (Optional) If specified, log probabilities for each token position will be returned.
+        :param tool_config: (Optional) Configuration for tool use.
         :returns: If stream=False, returns a ChatCompletionResponse with the full completion.
                  If stream=True, returns an SSE event stream of ChatCompletionResponseStreamChunk
         """