fix: remove unsupported outputMimeType and add response_modalities fo…

[mrdotb]

Description

The image generation with gemini is failing. Gemini does not support output format. Ref
I removed the code and implemented the response_modalities
I am not sure if it was ever supported by gemini or if it's something adapted from open ai.

ReqLLM.generate_image(
          "google:gemini-2.5-flash-image",
          "A futuristic cityscape with flying cars and neon lights",
          aspect_ratio: "3:4")
** (MatchError) no match of right hand side value:

    {:error,
     %ReqLLM.Error.API.Request{
       reason: "Provider response error (400): Google API error: Invalid JSON payload received. Unknown name \"outputMimeType\" at 'generation_config.image_config': Cannot find field.",
       status: 400,
       response_body: %{
         "error" => %{
           "code" => 400,
           "details" => [
             %{
               "@type" => "type.googleapis.com/google.rpc.BadRequest",
               "fieldViolations" => [
                 %{
                   "description" => "Invalid JSON payload received. Unknown name \"outputMimeType\" at 'generation_config.image_config': Cannot find field.",
                   "field" => "generation_config.image_config"
                 }
               ]
             }
           ],
           "message" => "Invalid JSON payload received. Unknown name \"outputMimeType\" at 'generation_config.image_config': Cannot find field.",
           "status" => "INVALID_ARGUMENT"
         }
       }

Type of Change

• Bug fix (non-breaking change fixing an issue)
• New feature (non-breaking change adding functionality)
• [] Breaking change (fix or feature causing existing functionality to change)
• Documentation update

Testing

• Tests pass (mix test)
• Quality checks pass (mix quality)

Checklist

• My code follows the project's style guidelines
• I have updated the documentation accordingly
• My commits follow conventional commit format
• I have NOT edited CHANGELOG.md (it is auto-generated by git_ops)