Add image generation support to OpenRouterProvider

[billyfranklim1]

Summary

This PR adds ImageProvider support to OpenRouterProvider, allowing image generation through OpenRouter's unified API.

OpenRouter supports image generation via the /api/v1/chat/completions endpoint using the modalities parameter (docs), but OpenRouterProvider only implemented EmbeddingProvider and TextProvider, causing a LogicException when attempting Image::of()->generate(provider: Lab::OpenRouter).

Changes

• OpenRouterImageGateway — New gateway that sends image generation requests to OpenRouter's chat completions endpoint with modalities: ["image", "text"], parses base64 data URI image responses, and maps them to GeneratedImage instances.
• OpenRouterProvider — Now implements ImageProvider using the GeneratesImages and HasImageGateway traits. Adds defaultImageModel() (google/gemini-2.5-flash-image), defaultImageOptions() (maps size to image_config.aspect_ratio), and imageGateway().
• Tests — 4 unit tests covering: basic generation, aspect ratio config passthrough, empty response handling, and multiple images in response.

Usage

use Laravel\Ai\Image;
use Laravel\Ai\Enums\Lab;

$response = Image::of('A sunset over mountains')
    ->landscape()
    ->generate(provider: Lab::OpenRouter, model: 'google/gemini-2.5-flash-image');

$image = $response->firstImage();
$binary = $image->content();

Fixes #330

[pushpak1300]

Can you fix merge conflicts and refactor to new namespace we have. i'm marking this draft once ready please mark this for review.