fix(auth): auto-switch profile on genuine exhaustion (regression from #374's false-cascade fix)

[andrei-hasna]

Summary

The /config "Auth profile auto-switch" toggle no longer switches profiles when the current profile is genuinely exhausted (real usage-limit / 429, e.g. "You've hit your usage limit, try again Jul 28"). The turn just parks on a self-heal retry until the (days-away) reset. This restores the legitimate switch without reviving the cross-agent false-cascade that #374 fixed.

Root cause (regression from #374)

#374 (a31719336) stopped a false weekly-limit cascade by making rolling account/rateLimits/updated snapshots display-only and gating all exhaustion bookkeeping + auto-switch behind the account-verified RateLimitSnapshotSource::AccountUsage read (codex-rs/tui/src/chatwidget/rate_limits.rs:314-339).

That over-corrected. Before #374 the rolling snapshot was also the trigger that drove auto-switch on genuine mid-turn exhaustion: it populated auth_profile_auto_switch_snapshots_by_limit_id and called maybe_auto_switch_auth_profile_for_rate_limit. After #374, on genuine exhaustion:

• The switch map (auth_profile_auto_switch_snapshots_by_limit_id) is empty — the limit is crossed mid-turn, before the ~60s authoritative heartbeat re-reads it, and the heartbeat is then suppressed for the exhausted profile (auth_profile_popups.rs:585-591).
• Both genuine-exhaustion fallbacks read that empty map and bail: try_auth_profile_switch_after_reset_unavailable (usage_limit_reset/automatic.rs:225-240) and maybe_auto_switch_auth_profile_before_user_turn (rate_limits.rs:409).
• The usage-limit turn-error path only switched when manual_usage_limit_reset_is_active() was true (turn_runtime.rs on_rate_limit_error), so with auto-reset unavailable it just scheduled a self-heal retry for the reset instant — "it just stops."

#374's own PR body flagged the follow-ups this closes ("consider verifying target capacity before switching"; choose_profile_for_auto_switch picking Unknown blindly).

Fix

Drive auto-switch from the current profile's own, authoritative usage-limit / 429 turn failure, which is authoritative for the current profile (the app server rejected this profile's own turn) and cannot be produced by a sibling agent's identity-less rolling snapshot:

• New try_auth_profile_switch_for_usage_limit (rate_limits.rs): prefers a cached account-verified exhausted window; when none is cached, synthesizes the trigger window from the operator's enabled auto-switch config (weekly preferred, else 5h; None when both opted out so a disabled window is never switched on) and reuses the existing candidate selection (cycles to an Unknown-but-untried profile; bails — no loop — when every alternate is exhausted). The reset instant is parsed from the error text so repeated genuine failures get distinct trigger keys and can cycle across profiles.
• turn_runtime.rs on_rate_limit_error: the reset-unavailable branch now calls it (gated to genuine UsageLimit, not transient 429/overload), dropping the too-narrow manual_usage_limit_reset_is_active() requirement.
• usage_limit_reset/automatic.rs fallback_auth_profile_switch_after_reset_unavailable: also routes through it, so the auto-reset path recovers even when the authoritative read hasn't cached the exhausted window yet.

Preserves #374's no-false-cascade invariant

Rolling snapshots stay strictly display-only and never seed the switch map. The new switch is reachable only from on_rate_limit_error / the post-error reset fallback — never from on_rolling_rate_limit_snapshot — so a sibling agent's foreign snapshot still cannot trigger a switch.

Tests (tui/src/chatwidget/tests/status_and_layout.rs)

• genuine_usage_limit_error_auto_switches_without_a_cached_snapshot — the regression: exhausted current profile + auto-switch on + another profile configured + empty map -> switches.
• rolling_snapshot_alone_never_switches_and_never_seeds_the_switch_map — fix(auth): stop false-positive weekly-limit cascade across auth profiles #374 preserved: rolling 100% snapshot never switches and never seeds the switch map.
• genuine_usage_limit_error_does_not_loop_when_all_profiles_exhausted — graceful: all alternates exhausted -> no switch, no loop (twice).

Verification (Blacksmith, 16 vCPU)

• Full codex-tui nextest suite: 3478 passed, 4 skipped, 0 failed.
• 3 new tests: PASS.
• cargo fmt --all -- --config imports_granularity=Item --check: clean (exit 0).
• cargo clippy -p codex-tui --tests --all-targets -- -D warnings: clean (exit 0; only a pre-existing transitive-dep proc-macro-error2 future-incompat note, unrelated).

Do not merge/tag yet — hand-off for 0.1.78.