Treat responses to collapsed requests as fresh (#1927)

Treat responses to collapsed requests as fresh (#1927)

Squid transactions involving collapsed requests receive their responses
as Store cache hits. Cache hits undergo mandatory freshness validation
checks and, if those checks detect a stale response, may trigger
revalidation (e.g., an If-Modified-Since request to the origin server).
This logic results in a still-being-delivered-to-Squid response
triggering its own revalidation if that response is deemed stale on
arrival by collapsed request (e.g., has a Cache-Control: max-age=0
header field).

HTTP RFC 9111 Section 4.7 briefly mentions collapsed requests but is
ambiguous with regard to validation of responses to collapsed requests.
IETF HTTP Working Group chair believes that specs are unclear, and that
these responses should not be treated as cache hits (in primary cases):
https://lists.w3.org/Archives/Public/ietf-http-wg/2024JanMar/0095.html

This change follows the above advice and prevents arguably excessive
freshness checks for responses to collapsed requests. This change is
collapsed-forwarding specific: It does not prevent freshness checks for
responses that were, at the time of a hit request, either fully cached
or still receiving response body bytes.

After this change, clientReplyContext can no longer collapse twice, once
after initial Store lookup and then again during refresh, because the
first collapse now precludes refresh.