gpt-oss : add support for response_format

[aldehir]

issue: ggml-org#15276

https://cookbook.openai.com/articles/openai-harmony#structured-output

Example:

curl http://192.168.48.110:11434/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "response_format": {
      "type": "json_object",
      "schema": {
        "$schema": "http://json-schema.org/draft-07/schema#",
        "title": "City Details",
        "description": "A simple object containing key details about a city.",
        "type": "object",
        "properties": {
          "country": {
            "description": "The country where the city is located.",
            "type": "string"
          },
          "landmarks": {
            "description": "A list of notable landmarks in the city.",
            "type": "array",
            "items": {
              "type": "string"
            }
          }
        },
        "required": ["country","landmarks"]
      }
    },
    "messages": [
      {
        "role": "system",
        "content": [
          {
            "type": "text",
            "text": "You are a helpful assistant designed to output JSON. For a given city, provide its country, and a list of three notable landmarks."
          },
          {
            "type": "text",
            "text": "# Response Formats\n## city\n### {\"type\": \"object\", \"properties\": { \"country\": { \"description\": \"The country where the city is located.\", \"type\": \"string\" }, \"landmarks\": { \"description\": \"A list of notable landmarks in the city.\", \"type\": \"array\", \"items\": { \"type\": \"string\" } } }, \"required\": [\"country\",\"landmarks\"] }"
          }
        ]
      },
      {
        "role": "user",
        "content": "Zürich"
      }
    ]
  }'

Output:

{
  "choices": [
    {
      "finish_reason": "stop",
      "index": 0,
      "message": {
        "role": "assistant",
        "reasoning_content": "User wants a JSON output. Format: type object with properties: country string, landmarks array of strings. Provide for Zürich: country Switzerland, landmarks: Swiss National Museum, Fraumünster, Lindenhof or maybe Lake Zurich? Provide three notable landmarks. Ensure JSON.",
        "content": "{\"country\":\"Switzerland\",\"landmarks\":[\"Swiss National Museum\",\"Fraumünster Church\",\"Lindenhof\"]}"
      }
    }
  ],
  "created": 1755059568,
  "model": "gpt-oss-20b",
  "system_fingerprint": "b6148-bf598f66",
  "object": "chat.completion",
  "usage": {
    "completion_tokens": 91,
    "prompt_tokens": 192,
    "total_tokens": 283
  },
  "id": "chatcmpl-syC14e0H640RVaUgMO1yFSWjZxgt4Ehe",
  "__verbose": {
    "index": 0,
    "content": "<|channel|>analysis<|message|>User wants a JSON output. Format: type object with properties: country string, landmarks array of strings. Provide for Zürich: country Switzerland, landmarks: Swiss National Museum, Fraumünster, Lindenhof or maybe Lake Zurich? Provide three notable landmarks. Ensure JSON.<|end|><|start|>assistant<|channel|>final <|constrain|>city<|message|>{\"country\":\"Switzerland\",\"landmarks\":[\"Swiss National Museum\",\"Fraumünster Church\",\"Lindenhof\"]}",
    "tokens": [],
    "id_slot": 1,
    "stop": true,
    "model": "gpt-oss-20b",
    "tokens_predicted": 91,
    "tokens_evaluated": 192,
    "generation_settings": {
      "grammar": "analysis ::= \"<|channel|>analysis<|message|>\" ( [^<] | \"<\" [^|] )* \"<|end|>\"\nchar ::= [^\"\\\\\\x7F\\x00-\\x1F] | [\\\\] ([\"\\\\bfnrt] | \"u\" [0-9a-fA-F]{4})\nconstrain ::= \"<|constrain|>\"? [a-zA-Z0-9_-]+\nfinal ::= \"<|channel|>final\" ( \" \" constrain )? \"<|message|>\" response\nresponse ::= \"{\" space response-country-kv \",\" space response-landmarks-kv \"}\" space\nresponse-country-kv ::= \"\\\"country\\\"\" space \":\" space string\nresponse-landmarks ::= \"[\" space (string (\",\" space string)*)? \"]\" space\nresponse-landmarks-kv ::= \"\\\"landmarks\\\"\" space \":\" space response-landmarks\nroot ::= analysis? \"<|start|>assistant\" final\nspace ::= | \" \" | \"\\n\"{1,2} [ \\t]{0,20}\nstring ::= \"\\\"\" char* \"\\\"\" space\n",
      "grammar_lazy": false,