fix(throttling-manager): Correct the backoff for concurrent 429s and zero Retry-After

[Mantisus]

Description

• Only the first 429 of a burst now advances the exponent, and it decays once the domain has left us alone for a full extra backoff window. Previously every concurrent 429 stepped it, so eight in-flight requests pushed the delay from 2s to past max_delay in one go, and any handled request reset the counter, including one that had just failed for good. throttled_until split into separate backoff_until and crawl_delay_until clocks, since the crawl delay is armed on every dispatch and would otherwise pass for an active backoff.

Issues

• Closes: ThrottlingRequestManager computes the wrong backoff for concurrent 429s and for Retry-After: 0 #2125

Testing

• Added new unit tests