[BUG] Gemini models does not support JSON mode

[drwl]

Basic checks

• I searched existing issues - this hasn't been reported
• I can reproduce this consistently
• This is a RubyLLM bug, not my application code

What's broken?

with_params(response_format: { type: 'json_object' }) does not work for Gemini models.

code:072> RubyLLM.chat(model: 'gemini-2.5-flash-lite').with_params(response_format: { type: 'json_object' }).ask('Testing. Respond with an json object with success = true')
code:72:in '<main>': Invalid JSON payload received. Unknown name "response_format": Cannot find field. (RubyLLM::BadRequestError)

However, setting the response mime type in the generator configuration works:
https://ai.google.dev/gemini-api/docs/structured-output?example=recipe#json_schema_support

chat:075> RubyLLM.chat(model: 'gemini-2.5-flash-lite').with_params(generationConfig: { responseMimeType: 'application/json' }).ask('Testing. Respond with a valid JSON with success = true')
=> 
#<RubyLLM::Message:0x00000001299f3628
 @content=#<RubyLLM::Content:0x0000000128daa420 @attachments=[], @text="{\n  \"success\": true\n}">,
 @model_id="gemini-2.5-flash-lite",
 @role=:assistant,
 @thinking=nil,
 @tokens=#<RubyLLM::Tokens:0x00000001299f3538 @cache_creation=nil, @cached=nil, @input=12, @output=9, @thinking=nil>,
 @tool_call_id=nil,
 @tool_calls=nil>

How to reproduce

1. RubyLLM.chat(model: 'gemini-2.5-flash-lite').with_params(response_format: { type: 'json_object' }).ask('Testing. Respond with an json object with success = true')
2. Errors

Expected behavior

If the gemini provider sees response_format: { type: 'json_object' }, it should either:

• Read this and update the provider specific configuration OR
• Raise an explicit error OR
• Clearly document in RubyLLM/docs that Gemini needs generationConfig: { responseMimeType: ..., responseSchema: ... } not response_format.

What actually happened

It's not usable

Environment

• Ruby version: (any)
• RubyLLM version: (latest)
• Provider: gemini
• OS: (any)