Pass through provider specific Gemini image aspect ratios

[morcken]

Gemini supports image aspect ratios such as 3:4, but the SDK currently drops any ratio outside 1:1, 2:3, and 3:2 when building Gemini image options.

This PR makes Gemini consistent with the existing size(string $size) API by passing through non-null size values as aspect_ratio, while preserving the existing mappings for 1:1, 2:3, and 3:2.

Reproduction

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

return Image::of($prompt)
    ->quality('medium')
    ->size('3:4')
    ->timeout(60)
    ->generate(provider: Lab::Gemini);

Before

• GeminiProvider::defaultImageOptions() converted unknown aspect ratios to null, so size('3:4') was not forwarded to Gemini.

After

• Non-null size values are passed through to Gemini as aspect_ratio.

Why this change

• Gemini supports 3:4 and other aspect ratios (https://ai.google.dev/gemini-api/docs/image-generation#3.1-flash-image-preview).
• size() already accepts a string at the public API level.
• OpenAI already forwards provider-specific size values instead of narrowing them to the three built-in ratios.
• This makes provider behavior more consistent without changing the convenience helpers.

Tests

Added a focused unit test that verifies:

• Gemini preserves 3:4 as aspect_ratio
• An existing mapped ratio (1:1) still behaves as before

Backward Compatibility

This change is backward compatible:

• No method signatures changed
• square(), portrait(), and landscape() are unchanged
• Existing known Gemini mappings are unchanged