Fix deprecated Gemini image model references

[grantf04]

Fixes #634
/claim #634

Summary

• remove retired gemini-2.0-flash-preview-image-generation and gemini-2.5-flash-image-preview from the TypeScript SDK Gemini model union/list
• drop the removed 2.0 image model from the Gemini provider smoke-test skip list
• update the Next image template to call the current gemini-2.5-flash-image model

Verification

• pnpm install (repo prepare build completed successfully; the example postinstall has an existing swallowed npm dedupe workspace-protocol warning)
• pnpm -C packages/sdk/ts run type-check
• pnpm --filter @merit-systems/echo-typescript-sdk run build
• pnpm -C packages/sdk/ts run test -- --runInBand
• pnpm 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

Note: templates/next-image is not included in pnpm-workspace.yaml, so pnpm -C templates/next-image run build cannot run from the root install because next is not installed for that standalone template.

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[grantf04]

The failing Vercel statuses are fork-deploy authorization prompts from the Merit Systems team, not code/test failures. Local verification from the PR body is green for the affected SDK path: install/prepare build, SDK type-check, SDK build, SDK tests, and Prettier check all completed locally.