fix(providers): append new models from GitHub catalog in enrichCodexModelsFromGithubCatalog (#15525)

[alltomatos]

Closes #15525

Summary

• In \enrichCodexModelsFromGithubCatalog, append official GitHub catalog models that are absent from live ChatGPT account model responses (\missingFromLive).
• Prevents newly released OpenAI/Codex models published in \models.json\ from being dropped when discovery is performed before the account's live endpoint surfaces them.
• Adds comprehensive unit test in \ ests/unit/codex-discovery-github-catalog-append.test.ts.