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

docs/resources/llama-stack-spec.html

@@ -4503,10 +4503,6 @@
                                 }
                             ]
                         }
-                    },
-                    "tool_prompt_format": {
-                        "$ref": "#/components/schemas/ToolPromptFormat",
-                        "default": "json"
                     }
                 },
                 "additionalProperties": false,
@@ -6522,10 +6518,6 @@
                                 }
                             ]
                         }
-                    },
-                    "tool_prompt_format": {
-                        "$ref": "#/components/schemas/ToolPromptFormat",
-                        "default": "json"
                     }
                 },
                 "additionalProperties": false,

docs/resources/llama-stack-spec.yaml

@@ -2600,9 +2600,6 @@ components:
           type: string
         tool_host:
           $ref: '#/components/schemas/ToolHost'
-        tool_prompt_format:
-          $ref: '#/components/schemas/ToolPromptFormat'
-          default: json
         toolgroup_id:
           type: string
         type:
@@ -2704,9 +2701,6 @@ components:
           items:
             $ref: '#/components/schemas/ToolParameter'
           type: array
-        tool_prompt_format:
-          $ref: '#/components/schemas/ToolPromptFormat'
-          default: json
       required:
       - name
       type: object