fix(copilot): add gpt-5.3-codex fallback model

[hyfdev]

Summary

• add a Copilot-side fallback that injects gpt-5.3-codex when models.dev has not rolled it out yet
• align fallback metadata with model release (release_date/last_updated = 2026-02-05)
• preserve existing Copilot behavior for costs and npm provider wiring

Upstream model catalog PR: anomalyco/models.dev#857

Closes#12954

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found.

[Copilot]

Pull request overview

This PR adds a client-side fallback mechanism for the gpt-5.3-codex model in the GitHub Copilot plugin, ensuring the model is available even before the upstream models.dev catalog includes it. The implementation creates a fallback model by copying properties from existing Codex models (prioritizing gpt-5.2-codex, then 5.1-codex-max, 5.1-codex, and 5.1-codex-mini) and overriding key fields.

Changes:

• Added ensureGpt53Codex function to inject gpt-5.3-codex model when missing from provider catalog
• Integrated the fallback into the Copilot auth loader to ensure model availability
• Aligned release date with official model announcement (2026-02-05)

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[hyfdev]

anomalyco/models.dev#857 is merged. Guess we could just wait for its release.