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
