fix: write RFC 3339 timestamps to codex auth.json

[Copilot]

Codex CLI changed its auth.json schema to expect RFC 3339 date-time strings for expires_at and last_refresh. bub login openai was writing raw Unix integers, causing codex to fail on startup with:

invalid type: integer `1776414280`, expected an RFC 3339 formatted date and time string at line 11 column 28

Changes

• save_openai_codex_oauth_tokens: writes expires_at and last_refresh as RFC 3339 strings ("2026-04-17T08:24:40Z") instead of integers
• _parse_tokens: accepts both RFC 3339 strings and legacy integer timestamps for backward compatibility when reading existing files
• Helpers added: _unix_to_rfc3339(ts) and _rfc3339_to_unix(value) for clean conversion in both directions

Resulting auth.json structure:

{
  "tokens": {
    "access_token": "...",
    "refresh_token": "...",
    "expires_at": "2026-04-17T08:24:40Z"
  },
  "last_refresh": "2026-04-17T08:56:01Z"
}