fix(ai-gateway): route Laguna S through OpenRouter only

[chrarnoldus]

Summary

• remove the Vercel model ID mapping for poolside/laguna-s-2.1:free
• keep the OpenRouter ID unchanged so it no longer matches the Vercel -free model entry
• add a regression assertion for the OpenRouter-only routing behavior

Verification

• pnpm --filter web test -- --runInBand src/lib/ai-gateway/providers/vercel/mapModelIdToVercel.test.ts
• pnpm --filter web lint
• pnpm --filter web typecheck
• git diff --check

Verification ran successfully with Node 26.5.0; the repository declares Node >=24 <25.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Executive Summary

Clean, self-consistent revert of the poolside/laguna-s-2.1:free Vercel model-ID mapping plus a matching regression test; verified with high confidence that no other code path still depends on the removed entry.

Review Notes

• Removing the entry makes mapModelIdToVercel('poolside/laguna-s-2.1:free') fall through to the kilo-exclusive lookup (no match — the ID is not registered in kiloExclusiveModels) and then to inferVercelFirstPartyInferenceProviderForModel, which returns null because poolside is absent from modelPrefixToVercelInferenceProviderMapping. The ID is therefore returned unchanged, so the new assertion holds.
• Downstream consumers (byok/index.ts, byok-router.ts, sync-providers.ts, providers/vercel/index.ts) look the mapped ID up in vercelModelMetadata; the unmapped ID will simply miss, which matches the stated intent of OpenRouter-only routing.
• Searched for poolside / laguna across the repo: no leftover references to poolside/laguna-s-2.1-free and no reverse Vercel→OpenRouter mapping to update.
• apps/web/src/lib/ai-gateway/AGENTS.md's forbiddenFreeModelIds rule does not apply — this ID is not a Kilo-exclusive free model, and the model itself is not being removed.

Files Reviewed (2 files)

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

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_{Reviewed by claude-opus-5 · Input: 26 · Output: 6.1K · Cached: 554.8K}

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