fix: don't pass default response format in Responses (#3614)

# What does this PR do?
Fireworks doesn't allow repsonse_format with tool use. The default
response format is 'text' anyway, so we can safely omit.


## Test Plan
Below script failed without the change, runs after.

```
#!/usr/bin/env python3
"""
Script to test Responses API with kubernetes-mcp-server.

This script:
1. Connects to the llama stack server
2. Uses the Responses API with MCP tools
3. Asks for the list of Kubernetes namespaces using the kubernetes-mcp-server
"""

import json

from openai import OpenAI

# Connect to the llama stack server
base_url = "http://localhost:8321/v1"
client = OpenAI(base_url=base_url, api_key="fake")

# Define the MCP tool pointing to the kubernetes-mcp-server
# The kubernetes-mcp-server is running on port 3000 with SSE endpoint at /sse
mcp_server_url = "http://localhost:3000/sse"

tools = [
    {
        "type": "mcp",
        "server_label": "k8s",
        "server_url": mcp_server_url,
    }
]

# Create a response request asking for k8s namespaces
print("Sending request to list Kubernetes namespaces...")
print(f"Using MCP server at: {mcp_server_url}")
print("Available tools will be listed automatically by the MCP server.")
print()

response = client.responses.create(
    # model="meta-llama/Llama-3.2-3B-Instruct",  # Using the vllm model
    model="fireworks/accounts/fireworks/models/llama4-scout-instruct-basic",
    # model="openai/gpt-4o",
    input="what are all the Kubernetes namespaces? Use tool call to `namespaces_list`. make sure to adhere to the tool calling format UNDER ALL CIRCUMSTANCES.",
    tools=tools,
    stream=False,
)

print("\n" + "=" * 80)
print("RESPONSE OUTPUT:")
print("=" * 80)

# Print the output
for i, output in enumerate(response.output):
    print(f"\n[Output {i + 1}] Type: {output.type}")
    if output.type == "mcp_list_tools":
        print(f"  Server: {output.server_label}")
        print(f"  Tools available: {[t.name for t in output.tools]}")
    elif output.type == "mcp_call":
        print(f"  Tool called: {output.name}")
        print(f"  Arguments: {output.arguments}")
        print(f"  Result: {output.output}")
        if output.error:
            print(f"  Error: {output.error}")
    elif output.type == "message":
        print(f"  Role: {output.role}")
        print(f"  Content: {output.content}")

print("\n" + "=" * 80)
print("FINAL RESPONSE TEXT:")
print("=" * 80)
print(response.output_text)
```

tests/integration/recordings/responses/73e97be515d9.json

@@ -0,0 +1,106 @@
+{
+  "request": {
+    "method": "POST",
+    "url": "http://0.0.0.0:11434/v1/v1/chat/completions",
+    "headers": {},
+    "body": {
+      "model": "llama3.2:3b-instruct-fp16",
+      "messages": [
+        {
+          "role": "user",
+          "content": "What's the weather in Tokyo? YOU MUST USE THE get_weather function to get the weather."
+        }
+      ],
+      "stream": true,
+      "tools": [
+        {
+          "type": "function",
+          "function": {
+            "type": "function",
+            "name": "get_weather",
+            "description": "Get the weather in a given city",
+            "parameters": {
+              "type": "object",
+              "properties": {
+                "city": {
+                  "type": "string",
+                  "description": "The city to get the weather for"
+                }
+              }
+            },
+            "strict": null
+          }
+        }
+      ]
+    },
+    "endpoint": "/v1/chat/completions",
+    "model": "llama3.2:3b-instruct-fp16"
+  },
+  "response": {
+    "body": [
+      {
+        "__type__": "openai.types.chat.chat_completion_chunk.ChatCompletionChunk",
+        "__data__": {
+          "id": "chatcmpl-116",
+          "choices": [
+            {
+              "delta": {
+                "content": "",
+                "function_call": null,
+                "refusal": null,
+                "role": "assistant",
+                "tool_calls": [
+                  {
+                    "index": 0,
+                    "id": "call_0c2qffvv",
+                    "function": {
+                      "arguments": "{\"city\":\"Tokyo\"}",
+                      "name": "get_weather"
+                    },
+                    "type": "function"
+                  }
+                ]
+              },
+              "finish_reason": null,
+              "index": 0,
+              "logprobs": null
+            }
+          ],
+          "created": 1759267492,
+          "model": "llama3.2:3b-instruct-fp16",
+          "object": "chat.completion.chunk",
+          "service_tier": null,
+          "system_fingerprint": "fp_ollama",
+          "usage": null
+        }
+      },
+      {
+        "__type__": "openai.types.chat.chat_completion_chunk.ChatCompletionChunk",
+        "__data__": {
+          "id": "chatcmpl-116",
+          "choices": [
+            {
+              "delta": {
+                "content": "",
+                "function_call": null,
+                "refusal": null,
+                "role": "assistant",
+                "tool_calls": null
+              },
+              "finish_reason": "stop",
+              "index": 0,
+              "logprobs": null
+            }
+          ],
+          "created": 1759267492,
+          "model": "llama3.2:3b-instruct-fp16",
+          "object": "chat.completion.chunk",
+          "service_tier": null,
+          "system_fingerprint": "fp_ollama",
+          "usage": null
+        }
+      }
+    ],
+    "is_streaming": true
+  }
+}