fix(circuit_breaker): clear rate keys on auto-reset

[levleontiev]

Summary

The circuit breaker's auto-reset path had a bug: when the cooldown elapsed and the state key was deleted, the rate accumulation keys were not cleared. For auto_reset_after_minutes < 2 (shorter than WINDOW_TTL_SECONDS = 120 s), the old high-rate data from the triggering spike was still alive in the shared dict. The sliding-window estimator would then read that stale data and immediately re-trip the breaker on the very first post-reset request — making auto-reset effectively a no-op.

Root cause

-- BEFORE (buggy): state key deleted, rate keys survive → breaker re-trips immediately
dict:delete(state_key)
-- falls through to rate check with stale previous_total still alive

Fix

Mirror what _M.reset() already does correctly — delete current and previous rate keys after clearing the state key:

-- AFTER: clear rate keys so sliding-window doesn't re-trip on first post-reset request
dict:delete(state_key)
local current_window_ar = floor(now / WINDOW_SIZE_SECONDS) * WINDOW_SIZE_SECONDS
local previous_window_ar = current_window_ar - WINDOW_SIZE_SECONDS
dict:delete(_M.build_rate_key(limit_key, current_window_ar))
dict:delete(_M.build_rate_key(limit_key, previous_window_ar))

Also introduces SECONDS_PER_MINUTE = 60 to decouple the elapsed-minutes calculation from WINDOW_SIZE_SECONDS — the two constants are coincidentally equal but represent different concepts.

Why tests didn't catch this

AC-4 uses auto_reset_after_minutes = 5 (300 s), well above WINDOW_TTL_SECONDS = 120 s, so rate keys expired naturally before auto-reset fired. The failure mode only manifests for auto_reset_after_minutes < 2.

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