feat: Add ModelsLab provider for image generation

[adhikjoshi]

Summary

Adds ModelsLab as a new provider for image generation in gen-ai Ruby gem.

What this PR adds

• New Provider: modelslab - Supports text-to-image generation via ModelsLab API
• Uses the Flux model by default
• Supports custom models, sizes, negative prompts, seeds, and other options

About ModelsLab

ModelsLab is a developer-first AI API platform providing:

• Text-to-image generation (Flux, Stable Diffusion, etc.)
• Text-to-video generation
• Voice/audio synthesis
• 3D generation
• Image editing

Usage Example

require 'gen_ai'

# Initialize ModelsLab image client
model = GenAI::Image.new(:modelslab, ENV['MODELSLAB_API_KEY'])

# Generate an image
result = model.generate('A sunset over mountains in watercolor style')
File.write('sunset.png', result.value)

# With custom options
result = model.generate('A cat sitting on a couch', 
  size: '512x512',
  num_inference_steps: 40,
  negative_prompt: 'blurry, low quality')

API Documentation

• ModelsLab Docs
• Image Generation API