Cache proactive auth refresh fallback

[cbusillo]

Summary

• record a cooldown when proactive ChatGPT token refresh fails but the cached access token is still valid
• keep expired tokens forcing refresh instead of falling back
• cover the retry-suppression behavior so immediate callers do not repeatedly hit the refresh endpoint during outages

Validation

• cargo test -p code-core proactive_refresh --lib
• cargo test -p code-core auth_for_stored_account --lib
• ./build-fast.sh

[shiny-code-bot]

Closing this PR as superseded by #223.

The auth fallback behavior from this PR was already landed in the refined replacement PR #223 (fix(core): cool down proactive auth refresh fallback). #223 keeps the useful cooldown behavior, adds the safer stored-account update_account_last_refresh path so fallback does not duplicate accounts, and is already merged into main.