Support Veo 3.1 Vertex AI video models

[SlaVKsVolks]

Summary:

• Adds Vertex AI supported-model entries for veo-3.1-fast-generate-preview and veo-3.1-generate-preview.
• Treats the new IDs as Veo 3-class models in VertexAIProvider.
• Updates the Next video template model types, request validation, selector options, and default model to Veo 3.1 Fast.

Fixes #595

Validation:

• pnpm -C packages/sdk/ts exec vitest run src/supported-models/video/vertex-ai.test.ts
• pnpm -C packages/sdk/ts type-check
• pnpm -C packages/sdk/ts exec eslint src/supported-models/video/vertex-ai.ts src/supported-models/video/vertex-ai.test.ts
• pnpm -C packages/sdk/ts exec tsup
• npm exec -- tsc --noEmit in templates/next-video-template
• npm exec -- eslint src/lib/types.ts src/app/api/generate-video/validation.ts src/app/api/generate-video/vertex.ts src/components/video-generator.tsx in templates/next-video-template
• git diff --check

Notes:

• Root pnpm install --frozen-lockfile populates dependencies, then the repo prepare lifecycle fails on Windows because it runs SKIP_ENV_VALIDATION=true ... through cmd.exe.
• Server package type-check is blocked before this patch by missing generated Prisma files; server lint is blocked before linting by __dirname in packages/app/server/eslint.config.mjs ESM scope.
• Template npm run build reaches Next.js but is blocked by a pre-existing missing zod / zod/v4 dependency in the installed template dependency graph. The changed template files pass direct TypeScript and ESLint checks.

[vercel]

@SlaVKsVolks is attempting to deploy a commit to the Merit Systems Team on Vercel.

A member of the Team first needs to authorize it.

[SlaVKsVolks]

CI/deploy checks are currently action_required because the fork workflows/Vercel deploy need maintainer authorization. I included the local validation I was able to run in the PR body, including the focused SDK test, SDK type-check/build, template type-check, changed-file ESLint checks, and git diff --check.