fix: align Grok reasoning variants

[chrarnoldus]

Summary

• expose low/medium/high reasoning variants for Grok 4.5 while retaining the Grok 4.2 toggle
• stop rewriting Grok 4.20 to Vercel reasoning/non-reasoning model IDs
• simplify Vercel model ID mapping call sites and tests

Verification

• pnpm --filter web test -- --runInBand src/lib/ai-gateway/providers/vercel/mapModelIdToVercel.test.ts (38 tests passed)
• pnpm --filter web lint
• git diff --check
• pnpm typecheck --changes-only (reported no matching workspace packages)

Environment

• Commands emitted the existing Node engine warning because local Node is v26.5.0 while the repository requires Node 24.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Executive Summary

The mapModelIdToVercel signature simplification and Grok 4.2/4.5 reasoning-variant split are applied consistently across all call sites and covered by updated tests, with no correctness issues found in the changed lines.

Files Reviewed (9 files)

• apps/web/src/app/api/openrouter/embeddings/route.ts
• apps/web/src/lib/ai-gateway/byok/index.ts
• apps/web/src/lib/ai-gateway/providers/model-settings.ts
• apps/web/src/lib/ai-gateway/providers/openrouter/sync-providers.ts
• apps/web/src/lib/ai-gateway/providers/vercel/index.ts
• apps/web/src/lib/ai-gateway/providers/vercel/mapModelIdToVercel.test.ts
• apps/web/src/lib/ai-gateway/providers/vercel/mapModelIdToVercel.ts
• apps/web/src/lib/ai-gateway/providers/xai.ts
• apps/web/src/routers/byok-router.ts

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_{Reviewed by claude-sonnet-5 · Input: 30 · Output: 8.4K · Cached: 749K}

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