Add support for ByteDance's LLM doubao-seed-1-6-thinking

Expected Behavior

Can use Spring AI to integrate with ByteDance's LLM doubao-seed-1-6-thinking-250715

or which starter should I choose?

Current Behavior

I tried using spring-ai-starter-model-openai, but the request parameters do not support the thinking field, and the response data also doesn’t include reasoning_content.

I hope Spring AI can support integration with more LLMs.

Request Example:

{
    "model": "doubao-seed-1-6-thinking-250715",
    "messages": [
      {
        "role": "user",
        "content": "Hello"
      },
      {
        "role": "system",
        "content": "You're an intelligent assistant."
      }
    ],
    "stream": true,
    "thinking": {
      "type": "enabled"
    },
    "stream_options": {
      "include_usage": true
    }
  }

Response Example:

...
{"choices":[{"delta":{"content":"","reasoning_content":" need","role":"assistant"},"index":0}],"created":1756883562,"id":"021756883562269078532ccda42bfe607645d235a4671675eb216","model":"doubao-seed-1-6-thinking-250715","service_tier":"default","object":"chat.completion.chunk","usage":null}

{"choices":[{"delta":{"content":"","reasoning_content":".","role":"assistant"},"index":0}],"created":1756883562,"id":"021756883562269078532ccda42bfe607645d235a4671675eb216","model":"doubao-seed-1-6-thinking-250715","service_tier":"default","object":"chat.completion.chunk","usage":null}

{"choices":[{"delta":{"content":"Hello","role":"assistant"},"index":0}],"created":1756883562,"id":"021756883562269078532ccda42bfe607645d235a4671675eb216","model":"doubao-seed-1-6-thinking-250715","service_tier":"default","object":"chat.completion.chunk","usage":null}

{"choices":[{"delta":{"content":"!","role":"assistant"},"index":0}],"created":1756883562,"id":"021756883562269078532ccda42bfe607645d235a4671675eb216","model":"doubao-seed-1-6-thinking-250715","service_tier":"default","object":"chat.completion.chunk","usage":null}

{"choices":[{"delta":{"content":" How","role":"assistant"},"index":0}],"created":1756883562,"id":"021756883562269078532ccda42bfe607645d235a4671675eb216","model":"doubao-seed-1-6-thinking-250715","service_tier":"default","object":"chat.completion.chunk","usage":null}

...

{"choices":[],"created":1756883562,"id":"021756883562269078532ccda42bfe607645d235a4671675eb216","model":"doubao-seed-1-6-thinking-250715","object":"chat.completion.chunk","usage":{"completion_tokens":72,"prompt_tokens":20,"total_tokens":92,"prompt_tokens_details":{"cached_tokens":0},"completion_tokens_details":{"reasoning_tokens":63}}}

[DONE]

Start with the spring-ai-starter-model-openai integration path, focusing on request parameters and streaming response handling. Compare the provided doubao-seed-1-6-thinking-250715 request and response examples with the current behavior; done means the thinking field can be sent and reasoning_content is exposed in responses.