Support Google Nano Banana 2 Lite image model (gemini-3.1-flash-lite-image)

[tombeckenham]

Update the Gemini adapter to support the two newly-released Google models: Gemini Omni Flash (video) and Nano Banana 2 Lite (image).

Scope split

Live-API verification (2026-07-01) showed the two models need very different work, so they've been split:

• This issue (Support Google Nano Banana 2 Lite image model (gemini-3.1-flash-lite-image) #870) — Nano Banana 2 Lite (gemini-3.1-flash-lite-image): a native Gemini image model that behaves exactly like the existing gemini-3.1-flash-image-preview (generateContent, inline image output, template-literal sizes). Small, low-risk registration.
• Follow-up Support Gemini Omni Flash (gemini-omni-flash-preview) via the Interactions API #871 — Gemini Omni Flash (gemini-omni-flash-preview): a video model that requires a brand-new Interactions API path (it rejects both generateContent and Veo's predictLongRunning). Tracked separately with the full verified design.

Nano Banana 2 Lite — verified facts

• Model id: gemini-3.1-flash-lite-image
• Method: generateContent (native path) → returns inline image/jpeg; ~1120 image tokens per 1K image
• Inputs: text + image (image-conditioned generation); Output: text + image
• Sizes: template-literal aspectRatio_resolution (e.g. 1:1_2K), 1K/2K/4K tiers
• Pricing: input $0.25 / 1M tokens, output $1.50 / 1M tokens ($0.0336 per 1K image)
• Positioned as the ultra-low-latency, low-cost tier of the Nano Banana 2 family (~4s/image)

Implementation (this issue)

• packages/ai-gemini/src/model-meta.ts: add GEMINI_3_1_FLASH_LITE_IMAGE + register in GEMINI_IMAGE_MODELS
• packages/ai-gemini/src/image/image-provider-options.ts: add to the GeminiNativeImageModels union (template-literal sizes + image input modalities)
• Unit test asserting native generateContent routing + size parsing
• Changeset, docs (docs/adapters/gemini.md, docs/media/image-generation.md), and media-generation skill model lists