fix: enable defs references in tool calls

[drbh]

This PR enables $defs to be set inside of function parameters

example:

import requests
import json

url = "http://localhost:3000/v1/chat/completions"

payload = {
    "messages": [
        {
            "content": "Classify the weather: It's sunny outside with clear skies",
            "role": "user",
        }
    ],
    "model": "test-model",
    "tools": [
        {
            "type": "function",
            "function": {
                "name": "classify_weather",
                "description": "Classify weather conditions",
                "parameters": {
                    "$defs": {
                        "WeatherType": {
                            "enum": ["sunny", "cloudy", "rainy", "snowy"],
                            "type": "string",
                        }
                    },
                    "properties": {"weather": {"$ref": "#/$defs/WeatherType"}},
                    "required": ["weather"],
                    "type": "object",
                },
            },
        }
    ],
}

response = requests.post(url, json=payload)

print(json.dumps(response.json(), indent=2))

expected response

{
  "object": "chat.completion",
  "id": "",
  "created": 1751895703,
  "model": "test-model",
  "system_fingerprint": "3.3.4-dev0-native",
  "choices": [
    {
      "index": 0,
      "message": {
        "role": "assistant",
        "tool_calls": [
          {
            "id": "0",
            "type": "function",
            "function": {
              "description": null,
              "name": "classify_weather",
              "arguments": "{\"weather\":\"sunny\"}"
            }
          }
        ]
      },
      "logprobs": null,
      "finish_reason": "stop"
    }
  ],
  "usage": {
    "prompt_tokens": 196,
    "completion_tokens": 18,
    "total_tokens": 214
  }
}