docs(ai-gateway): document image endpoints and GenerateImage reference images

[wixysam]

Summary

• AI Gateway module JSDoc: adds an Images section covering /images/generations and /images/edits, and a Vercel AI SDK generateImage example. It links to a new guide page in the docs. The intro no longer describes the gateway as Chat Completions only.
• GenerateImageParams: adds existing_image_urls?: string[]. The backend has accepted it since apper #1827 (2025-12-15), but the SDK types didn't expose it. Adds an editing example and points to the gateway for model, aspect ratio, resolution and quality control.

JSDoc and types only, no runtime changes.

Docs

The generated ai-gateway and integrations pages, plus the new guide (developers/references/sdk/getting-started/ai-gateway-images), are in the paired mintlify-docs PR on the same branch name. The guide link in this JSDoc resolves once that PR merges.

Verification

• npx tsc --noEmit passes.
• npm run create-docs-local generates both pages, previewed locally with mintlify dev.
• The Vercel AI SDK behavior in the example (providerOptions.base44 merged into the request body) was checked against the @ai-sdk/openai-compatible 2.0.75 source. It hasn't been run against the live gateway.

🤖 Generated with Claude Code

[github-actions]

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