fix: remove retired Gemini image models

[richboyneedcash]

Summary

• remove the retired gemini-2.0-flash-preview-image-generation and gemini-2.5-flash-image-preview IDs from the TypeScript SDK Gemini model type/list
• update the next-image Google generate/edit handlers to use the stable gemini-2.5-flash-image model
• remove the retired generateText smoke-test blacklist entry so the blacklist only contains models still present in the SDK list

Verification

• rg -n "gemini-2\.0-flash-preview-image-generation|gemini-2\.5-flash-image-preview" . || true returns no matches
• npx --yes pnpm@10.11.0 -C packages/sdk/ts run type-check
• npx --yes pnpm@10.11.0 -C packages/sdk/ts run build
• npx --yes pnpm@10.11.0 exec prettier --check packages/sdk/ts/src/supported-models/chat/gemini.ts packages/tests/provider-smoke/gemini-generate-text.test.ts templates/next-image/src/app/api/edit-image/google.ts templates/next-image/src/app/api/generate-image/google.ts
• npx tsc --noEmit in templates/next-image after installing that non-workspace template with npm install --package-lock=false --legacy-peer-deps

/claim #634

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