fix(ai-gateway): use Poolside key for free Laguna

[chrarnoldus]

Summary

• route poolside/laguna-s-2.1:free through Vercel using the Vercel-specific model ID restored from fix(ai-gateway): route Laguna through Vercel #4977
• add the Poolside BYOK credential when no user BYOK is configured and POOLSIDE_FREE_API_KEY is set
• preserve user BYOK precedence and skip the fallback when the environment variable is unset
• document POOLSIDE_FREE_API_KEY

Testing

• 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 typecheck
• pnpm --filter web lint
• pnpm exec oxfmt --list-different apps/web/src/lib/ai-gateway/providers/vercel/index.ts apps/web/src/lib/ai-gateway/providers/vercel/index.test.ts apps/web/src/lib/ai-gateway/providers/vercel/mapModelIdToVercel.ts apps/web/src/lib/ai-gateway/providers/vercel/mapModelIdToVercel.test.ts ENVIRONMENT.md
• git diff --check origin/main...HEAD

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (5 files)

• ENVIRONMENT.md
• apps/web/src/lib/ai-gateway/providers/vercel/index.ts
• apps/web/src/lib/ai-gateway/providers/vercel/index.test.ts
• apps/web/src/lib/ai-gateway/providers/vercel/mapModelIdToVercel.ts
• apps/web/src/lib/ai-gateway/providers/vercel/mapModelIdToVercel.test.ts

Previous Review Summary (commit 17da342)

Current summary above is authoritative. Previous snapshots are kept for context only.

Previous review (commit 17da342)

Status: No Issues Found | Recommendation: Merge

Files Reviewed (3 files)

• apps/web/src/lib/ai-gateway/providers/vercel/index.ts
• apps/web/src/lib/ai-gateway/providers/vercel/index.test.ts
• ENVIRONMENT.md

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_{Reviewed by kimi-k3 · Input: 39.3K · Output: 6.9K · Cached: 228.4K}

_{Review guidance: REVIEW.md from base branch main}