test(cache): cover a late 304 and widen the stale margin

[jamiehdev]

Tests only. A new test covers the case where a background revalidation gets a 304 after a no-cache request has stored a newer response. The newer entry must stay. The stale-while-revalidate test now has a wider timing margin, so a loaded CI runner cannot flake it.

Changes

• late_not_modified_leaves_a_newer_entry_in_place delays the 304 by 2 s and waits on origin requests and the revalidations counter, not on sleeps.
• stale_while_revalidate_serves_stale_and_refreshes_in_the_background uses a 3 s origin delay and a 1 s bound. The old values were 500 ms and 400 ms.

Verification

• With the Arc::ptr_eq check in Store::freshen removed, the new test fails: the next hit carries the old "v1" ETag.
• With while_revalidating forced to revalidate before it answers, the stale test fails its elapsed-time assertion.
• Both tests passed 20 runs in a row.