Add an optional releases.openai.com installer source

[copyberry]

Add an optional releases.openai.com installer source

What changed

• Let the shell and PowerShell installers resolve metadata and download assets from releases.openai.com when CODEX_INSTALLER_USE_RELEASES_OPENAI_COM is enabled.
• Fall back to GitHub Releases when metadata or asset downloads from the alternate source are unavailable.
• Verify that an installed or cached Codex binary reports the resolved version before activating it.

Testing

• Cover latest-release installation, asset fallback, mismatched binary versions, and reuse of a cached legacy package in the shell installer tests.

[github-actions]

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