Implement automatic auth switching between ChatGPT and API key when u…

…sage limits are reached (#2)

* Initial plan

* Implement auto-switch to API credentials when ChatGPT usage is capped

Co-authored-by: iainlowe <3978+iainlowe@users.noreply.github.com>

* Update codex-rs/login/src/lib.rs

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update codex-rs/core/tests/auth_switching_integration.rs

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: iainlowe <3978+iainlowe@users.noreply.github.com>
Co-authored-by: Iain Lowe <iain.lowe@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>