Follow-up hardening on the cherry-picked pool-fallback fix. The original
_resolve_codex_usage_credentials wrapped BOTH resolve_codex_runtime_credentials()
and the separate _read_codex_tokens() account_id read in one broad
'except Exception: pass', which had three problems:
1. A transient refresh/network failure (non-AuthError) from the resolver was
silently swallowed and downgraded to pool.select(), which could report
/usage limits for a DIFFERENT pool account than the one actually running.
On main that error surfaced. This is a real behavior regression for the
multi-account/pool case.
2. If the resolver succeeded but only the account_id read raised, the whole
singleton tier was abandoned in favor of a pool token that carries no
ChatGPT-Account-Id header (PooledCredential has no account_id concept),
risking a wrong-account read or 401.
3. 'except Exception' masked genuine programming errors.
Fix: narrow the outer catch to AuthError (the documented 'no creds' failure
mode of both functions), and read account_id in a best-effort inner try so a
partial/missing singleton store can't sink an otherwise-usable credential.
Transient errors now propagate and fail open via the outer fetch_account_usage
guard rather than mis-routing to the wrong account. Adds debug breadcrumbs and
a comment characterizing when the tier-3 pool path actually fires.
Guard tests: a non-AuthError resolver failure must NOT swap to the pool
(fail-open, no snapshot); an account_id read failure keeps the singleton token.
Updated the existing pool-fallback test to use AuthError (the real failure
mode) instead of a generic RuntimeError.
