refactor(ai-gateway): clarify usage/generation cost mismatch warning

[chrarnoldus]

Rewords the dev-only warning in processTokenData that fires when the provider's generation lookup yields a different cost_mUsd than the response's own usage payload:

• Before: DEV ODDITY / WARNING: Usage stats do not match generation data: followed by two unlabeled [a, b] arrays.
• After: an explicit message stating what is being compared, with labeled values (generation vs response) for cost_mUsd and cacheDiscount_mUsd, plus a comment noting the generation lookup values win.

No behavior change; warning text and structure only.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Executive Summary

Pure logging/message rewording in processTokenData's cost-mismatch warning with no behavior change; no issues found.

Files Reviewed (1 files)

• apps/web/src/lib/ai-gateway/processUsage.ts

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_{Reviewed by claude-sonnet-5 · Input: 12 · Output: 1.6K · Cached: 197.6K}

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