Reset ChatGPT Codex auth during OAuth setup

[vincenzopalazzo]

Summary

• force chatgpt_codex reconfiguration to start a fresh OAuth flow instead of silently reusing cached tokens
• restore the previous cached token if the new OAuth attempt fails or is cancelled
• register chatgpt_codex cleanup so deleting the provider clears cached OAuth state

Review Notes

• Production-safety review passed (3 rounds)
• Focus area: explicit re-auth should let users switch ChatGPT subscriptions without breaking existing auth on failed relogin

Test plan

• cargo test -p goose chatgpt_codex --no-default-features --features rustls-tls -- --nocapture
• cargo clippy -p goose --no-default-features --features rustls-tls --lib -- -D warnings
• CI passes
• Reconfigure ChatGPT Codex after existing login and verify browser flow prompts for a fresh login
• Cancel reconfigure midway and verify the previous login still works

[chatgpt-codex-connector]

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