fix: Simplify Groq tool_choice to String

[DavertMik]

I couldn't make Groq adapter tool choice work with current schema:

Currently it changes:

tool_choice: 'auto' 

to

tool_choice: {}

So I receive this response

 Sending: POST http://api.groq.com:443/openai/v1/chat/completions
D, [2025-11-02T23:32:55.482715 #1278525] DEBUG -- : [httplog] Data: {"model":"openai/gpt-oss-20b","tool_choice":{},

[httplog] Response:
{"error":{"message":"'tool_choice' : value must be a string (auto or none) or an object like `{'type': 'function', 'function': {'name': 'my_function'}}`","type":"invalid_request_error"}}

I tried to pick up the OpenAI definition:

          tool_choice               arguments: :type do 
            type                    Symbol, in: [ :none, :auto, :required ]
            function                arguments: :name do 
              name                  Symbol 
            end 
          end 

But it constructed object which again was not consumed by Groq API

"tool_choice":{"type":"auto"}
{"error":{"message":"'tool_choice' : value must be a string (auto or none) or an object like `{'type': 'function', 'function': {'name': 'my_function'}}`","type":"invalid_request_error"}}

I didn't find a way in dynamic schema to make it Sctring or Object so my proposed fix focuses on basic approaches:

• tool_choice: auto
• tool_choice: none

@kristoph I saw you used Kimi K2 f7e8e8c Can you also try it with a tool call (as it is agentic model)?