feat: add gpt-5.3-codex to github-copilot provider

[Avivskie]

Summary

• Adds gpt-5.3-codex model definition to the github-copilot provider.
• GPT-5.3-Codex was made generally available for GitHub Copilot on February 9, 2026: https://github.blog/changelog/2026-02-09-gpt-5-3-codex-is-now-generally-available-for-github-copilot/
• Model definition follows the same structure as the existing gpt-5.2-codex.toml, with updated release date and name.
• The OpenAI provider entry (providers/openai/models/gpt-5.3-codex.toml) already exists; this adds the corresponding GitHub Copilot entry with zero cost (subscription-based) and matching Copilot context limits (272K context / 128K output).

[timothyis]

2 PRs already exist for this. First open #993

[fwang]

thanks @Avivskie already added thru another PR