Retry account switches after usage limits

[cbusillo]

Summary

• retry a sampling request after switching saved accounts on typed UsageLimitReached responses
• keep runtime rate-limit switch state for the whole turn so exhausted ChatGPT accounts are not reselected across follow-up sampling rounds
• add limited ChatGPT account tracking and API-key fallback gating for no-reset/stale-reset limit responses

Good PRs passed on the way

• Add account switch usage foundation #209 usage foundation remains aligned
• Record account usage from runtime rate limits #210 runtime usage persistence remains aligned
• Auto-switch preferred account for new CLI sessions #211 new-session preferred account switching remains aligned

Aligns With #126

This closes the next auth/account-switching audit miss after #211: runtime retry switching on UsageLimitReached plus RateLimitSwitchState fallback semantics. Plain saved API-key-to-saved-API-key rotation remains a follow-up because cached API-key auth does not currently expose a saved account id through the same path.

Validation

• cargo check -p codex-core --lib
• cargo test -p codex-core account_switching::tests::limited_chatgpt_account_is_not_reselected_without_reset_hint --lib -- --exact
• cargo test -p codex-core account_switching::tests::limited_chatgpt_account_is_not_reselected_after_expired_reset_hint --lib -- --exact
• cargo test -p codex-core account_switching::tests::api_key_fallback_requires_all_chatgpt_accounts_marked_limited --lib -- --exact

Refs #126