/simplify-code pass on the Retry-After salvage stack:
- compute_error_backoff now decides "no usable cooldown" exactly once:
a parsed 0.0 (retry-after: 0, or an HTTP-date in the past, which the
shared parser clamps to 0) is treated as absent instead of falling
through an accidental falsy check — prevents a hot-loop retry and
keeps the sentinel semantics uniform (is None / is not None at all
four sites).
- Comment corrected: the sibling unwrap lives in
extract_api_error_context, not _extract_rate_limit_context.
- Test driver deduped: one _retryable_error factory + one _drive_once
shared by the 429 and 524 tests; added the over-cap (3600 → 600) and
no-cooldown fallback rows requested in the #103722 review.
Mutation checks: nested-unwrap neutralized → nested row red; cap
removed → over-cap row red; restored → all 298 green.
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