The circuit breaker ignored the trip-on-timeout, trip-on-4xx, and
trip-on-5xx flags and never cleared its counts, so the failure ratio
became lifetime-cumulative. The backoff-multiplier and max-backoff
settings did nothing. Circuit-open surfaced as HTTP 500.
Failure classification now honours the trip flags, the breaker uses a
rolling count window, and repeated trips extend the open state up to the
configured maximum. A helper maps circuit-open to HTTP 503. The request
body is copied once per request instead of once per retry. Defaults
reproduce the prior behaviour: the breaker is off by default and a
backoff multiplier of one keeps a constant open window.
IsRetryable and GetStatusCode used a direct type assertion, so a ProxyError
wrapped with %w was never detected (status fell back to 500, retryable to
false). Use errors.As, which dereferences the existing Unwrap chain and
matches the file's own Unwrap method.
Test: Test_IsRetryable_GetStatusCode_Wrapped (wrapped error preserves both
flags).
Note: these helpers currently have no production callers (test-only
surface), so this is a correctness hardening of the error API.
lukaszraczylo
