refactor(ai-gateway): isolate OpenRouter display pricing

[chrarnoldus]

Depends on #4859. This stacked PR must merge after its base PR.

Summary

• extract OpenRouter display-pricing helpers into a lightweight provider module
• import display pricing directly in the cached endpoints route
• preserve provider-index exports and GPT-5.6 promotion behavior
• move focused discount and promotion tests alongside the new module

Testing

• pnpm exec oxfmt on the five changed files
• pnpm --filter web test --runInBand --runTestsByPath ... (3 suites, 40 tests)
• pnpm --filter web lint -- ... (0 warnings, 0 errors)
• pnpm --filter web typecheck

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Executive Summary

This is a pure refactor extracting OpenRouter display-pricing helpers into a dedicated module, with backwards-compatible re-exports and no behavioral changes.

Files Reviewed (5 files)

• apps/web/src/app/api/openrouter/models/[provider]/[model]/endpoints/route.ts
• apps/web/src/lib/ai-gateway/providers/openrouter/display-pricing.ts
• apps/web/src/lib/ai-gateway/providers/openrouter/display-pricing.test.ts
• apps/web/src/lib/ai-gateway/providers/openrouter/index.ts
• apps/web/src/lib/ai-gateway/providers/openrouter/index.test.ts

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_{Reviewed by claude-sonnet-5 · Input: 30 · Output: 4.1K · Cached: 628.6K}

_{Review guidance: REVIEW.md from base branch add-openrouter-gateway-endpoints}