fix(ai-gateway): disable GPT-5.6 promotion

[chrarnoldus]

Summary

• disable the OpenRouter GPT-5.6 promotion
• prefer the openai inference provider for OpenAI GPT models independently of the promo flag
• update promo and provider-routing expectations for the disabled state

Testing

• not run locally; CI will validate the change

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Executive Summary

Flag flip to disable the GPT-5.6 promotion plus a widening of the preferred inference provider to all OpenAI models; verified the flag's three consumers (display-pricing.ts:26, openrouter/index.ts:90, vercel/index.ts:98) and the updated pricing/name expectations are arithmetically and behaviorally consistent, with high confidence.

Notes and assumptions (no action required):

• getPreferredProviderOrder now returns ['openai'] for every model matching isOpenAiModel (any id containing openai/gpt, excluding GPT-OSS), not just the four promo ids. This is stated as intentional in the PR description. It also changes catalog pricing selection in openrouter/index.ts:139-149, where the preferred-endpoint lookup has no endpoints[0] fallback; this matches the existing behavior for Claude/DeepSeek/GLM/etc., so it is not flagged. order is a preference (fallbacks still allowed) and only/ignore are untouched, so Vercel routing eligibility is unaffected, and openai maps to a valid Vercel provider slug.
• Kilo-exclusive stealth/gpt-5.6-sol also matches isOpenAiModel, but it is served through the martian gateway, so applyPreferredProvider (openrouter/vercel only) and the OpenRouter catalog mapping do not apply to it.
• With the flag off, the shouldRouteToVercel promo guard becomes unreachable and its only test was deleted rather than inverted; acceptable for a toggle, but the guard is now uncovered.
• Test expectations verified by hand: undoPricingDiscount at discount: 0.5 yields the asserted 0.000002000000/0.000008000000/0.000000200000, and prompt: '0.00001' hits the ($$$$) threshold in formatName, so the ($$$$) expectations are correct. Removed imports (jest, OPENROUTER_GPT56_PROMO_MODEL_IDS) are no longer referenced, and GatewayRequest is still used in vercel/index.test.ts.
• Tests were not executed (read-only review mode); CI should confirm.

Files Reviewed (6 files)

• apps/web/src/lib/ai-gateway/providers/openai.ts
• apps/web/src/lib/ai-gateway/providers/apply-provider-specific-logic.ts
• apps/web/src/lib/ai-gateway/providers/apply-provider-specific-logic.test.ts
• apps/web/src/lib/ai-gateway/providers/openrouter/display-pricing.test.ts
• apps/web/src/lib/ai-gateway/providers/openrouter/index.test.ts
• apps/web/src/lib/ai-gateway/providers/vercel/index.test.ts

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_{Reviewed by claude-opus-5 · Input: 70 · Output: 23.1K · Cached: 2.1M}

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