feat(ai-gateway): route fast Opus and GPT Pro through Vercel

[chrarnoldus]

Summary

• map the fast Claude Opus variant to its base Vercel model and set Anthropic speed: fast
• map GPT Pro variants to their base Vercel models and set OpenAI reasoningMode: pro
• centralize Anthropic and OpenAI variant detection in shared provider helpers
• extend Vercel provider-option types and cover option generation and model-ID mappings

Verification

• pnpm --filter web test -- --runInBand apps/web/src/lib/ai-gateway/providers/vercel/index.test.ts apps/web/src/lib/ai-gateway/providers/vercel/mapModelIdToVercel.test.ts
• pnpm --filter web test -- --runInBand apps/web/src/lib/ai-gateway/providers/vercel/index.test.ts apps/web/src/lib/ai-gateway/providers/openrouter/index.test.ts apps/web/src/lib/ai-gateway/providers/model-prefix.test.ts
• pnpm --filter web typecheck
• pnpm exec oxfmt --list-different apps/web/src/lib/ai-gateway/providers/vercel/mapModelIdToVercel.ts apps/web/src/lib/ai-gateway/providers/vercel/mapModelIdToVercel.test.ts
• git diff --check