feat(ai-gateway): sanitize custom LLM JSON refs

[chrarnoldus]

Summary

• add an optional sanitize_ref_fields custom LLM upstream setting
• recursively rename $ref keys to _ref in valid JSON Chat Completions tool results
• preserve invalid/non-matching JSON and leave Messages/Responses requests unchanged

Testing

• pnpm --filter web exec jest src/lib/ai-gateway/experiments/build-direct-provider.test.ts --runInBand
• pnpm --filter web run typecheck
• pnpm --filter web run lint
• pnpm --filter @kilocode/db run typecheck
• pnpm --filter @kilocode/db run lint
• pnpm format
• git diff --check

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (3 files)

• apps/web/src/lib/ai-gateway/experiments/build-direct-provider.ts
• apps/web/src/lib/ai-gateway/experiments/build-direct-provider.test.ts
• packages/db/src/schema-types.ts

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_{Reviewed by kimi-k3 · Input: 52.1K · Output: 10.6K · Cached: 534.9K}

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