fix(ai-gateway): defer model retirement labels

[chrarnoldus]

Summary

• Show the (retires ...) model-name suffix only when the expiration date is within one calendar month.
• Keep distant retirement dates hidden until they become timely, with deterministic boundary coverage.

Verification

• Not manually tested; this is isolated model-name formatting behavior.

Visual Changes

N/A

Reviewer Notes

The threshold uses a calendar month rather than a fixed number of days.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Executive Summary

Adds a one-calendar-month threshold to OpenRouter model retirement labels; the change is clean, well-tested with deterministic fake timers, and the date-fns addMonths import is already a project dependency.

Files Reviewed (2 files)

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