The stock provider only exposes 1:1, 2:3, 3:2 via defaultImageOptions()
match statement — any other ratio gets silently dropped to null.
ExtendedGeminiProvider adds 9:16, 16:9, 4:3, 3:4, 4:5, 5:4, 21:9 so
Image::of()->size('9:16') now works for Reels/Stories/TikTok/Shorts
and size('16:9') for X/LinkedIn/YouTube landscape content.
Gemini's native API supports all these ratios; the SDK restriction was
purely in the provider's match statement.
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