feat(ai-gateway): add discounted GPT-5.6 Sol model

[chrarnoldus]

Summary

• add openai/gpt-5.6-sol-discounted using the discounted OpenAI endpoint through Vercel AI Gateway
• restrict inference to OpenAI without data collection or stealth flags and disable the previous 20% off stealth model
• exclude the discounted model from explicit managed Vercel OpenAI BYOK and document that OpenAI BYOK must be disabled in the Vercel GUI
• the discounted promotion runs through September 18, 2026

Verification

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

Operations

• OpenAI BYOK must be disabled for this route in the Vercel AI Gateway GUI so requests use Vercel billing and retain the discounted endpoint pricing.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Executive Summary

Incremental change only adds a promotion end date to the discounted GPT-5.6 Sol description; no high-confidence defects.

Files Reviewed (1 files)

• apps/web/src/lib/ai-gateway/providers/openai-exclusive.ts

Previous Review Summary (commit b0cde35)

Current summary above is authoritative. Previous snapshots are kept for context only.

Previous review (commit b0cde35)

Status: No Issues Found | Recommendation: Merge

Executive Summary

Reviewed the discounted GPT-5.6 Sol exclusive model, stealth disablement, and Vercel managed-BYOK skip; no high-confidence defects in the current HEAD.

Files Reviewed (6 files)

• apps/web/src/lib/ai-gateway/models.test.ts
• apps/web/src/lib/ai-gateway/models.ts
• apps/web/src/lib/ai-gateway/providers/openai-exclusive.ts
• apps/web/src/lib/ai-gateway/providers/vercel/index.test.ts
• apps/web/src/lib/ai-gateway/providers/vercel/index.ts
• apps/web/src/tests/openrouter-models-config.test.ts

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