remove conflicting default for tool prompt format in chat completion (#742)

# What does this PR do?
We are setting a default value of json for tool prompt format, which
conflicts with llama 3.2/3.3 models since they use python list. This PR
changes the defaults to None and in the code, we infer default based on
the model.

Addresses: #695 

Tests:
❯ LLAMA_STACK_BASE_URL=http://localhost:5000 pytest -v
tests/client-sdk/inference/test_inference.py -k
"test_text_chat_completion"

 pytest llama_stack/providers/tests/inference/test_prompt_adapter.py

llama_stack/providers/remote/inference/fireworks/fireworks.py

@@ -8,7 +8,6 @@ from typing import AsyncGenerator, List, Optional, Union
 
 from fireworks.client import Fireworks
 from llama_models.datatypes import CoreModelId
-
 from llama_models.llama3.api.chat_format import ChatFormat
 from llama_models.llama3.api.tokenizer import Tokenizer
 
@@ -52,7 +51,6 @@ from llama_stack.providers.utils.inference.prompt_adapter import (
 
 from .config import FireworksImplConfig
 
-
 MODEL_ALIASES = [
     build_model_alias(
         "fireworks/llama-v3p1-8b-instruct",
@@ -198,7 +196,7 @@ class FireworksInferenceAdapter(
         sampling_params: Optional[SamplingParams] = SamplingParams(),
         tools: Optional[List[ToolDefinition]] = None,
         tool_choice: Optional[ToolChoice] = ToolChoice.auto,
-        tool_prompt_format: Optional[ToolPromptFormat] = ToolPromptFormat.json,
+        tool_prompt_format: Optional[ToolPromptFormat] = None,
         response_format: Optional[ResponseFormat] = None,
         stream: Optional[bool] = False,
         logprobs: Optional[LogProbConfig] = None,