fix(ai-gateway): mark GLM 5.2 free unavailable

[chrarnoldus]

Summary

• add z-ai/glm-5.2:free to the AI Gateway unavailable-model set
• ensure stale clients receive the generic unavailable-model response

Testing

• Not run locally; CI will handle validation as requested.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Executive Summary

Adds z-ai/glm-5.2:free to the AI Gateway unavailable-model set so stale clients get the generic unavailable response; paid z-ai/glm-5.2 remains available via exact-match gating.

Files Reviewed (1 files)

• apps/web/src/lib/ai-gateway/unavailable-models.ts

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