refactor: early-return writeResponseToCache on cache hit

[an-adamchuk]

Summary

Restructure AbstractUseStaleRequest::writeResponseToCache() to early-return when shouldWriteResponseToCache() is false, instead of wrapping only the refresh-marker write and then falling through to parent::writeResponseToCache().

Why

The previous shape was correct in net effect — the parent already short-circuits on cache hit via its own guard — but it had two drawbacks:

1. On every cache-hit invocation it still walked into parent::writeResponseToCache(), doing duplicate guard work that produced no side effects.
2. Subclasses overriding writeResponseToCache() got an unclear contract: calling parent:: looked like it might do something, but actually never did on a cache hit.

The new shape makes the no-op explicit. If a subclass calls parent::writeResponseToCache() and adds side effects after, the parent's behavior is now unambiguous: it either fully wrote (marker + response) or it didn't run at all.

This is motivated by debugging an AIS-side bug (MR-7089) where a subclass of AbstractUseStaleRequest added Cache::tags(...)->flush() after parent::writeResponseToCache(). The flush fired on every cache hit because nothing in the chain signalled "no write happened." Cleaner framework semantics make that class of mistake easier to spot.

What changed

• app/AbstractUseStaleRequest.php — writeResponseToCache() now returns at the top when the guard is false.
• tests/Feature/AbstractUseStaleRequestTest.php — three new tests covering the three branches:
  • testCacheHitDoesNotWriteRefreshMarker — seeds a sentinel marker, calls sync(), asserts the sentinel survives.
  • testFreshFetchWritesRefreshMarker — asserts the marker is written with value 'refresh after' after a live fetch.
  • testWriteCacheDisabledSkipsRefreshMarker — asserts setWriteCache(false) blocks both the marker write and the response write.

Behavior change

None at the framework boundary — all existing tests should still pass, including testCacheBehaviorUnderHeavyLoad which exercises the full lifecycle. The parent method simply isn't invoked on a cache hit anymore (it would have no-op'd anyway).

Test plan

• vendor/bin/phpunit tests/Feature/AbstractUseStaleRequestTest.php passes locally
• Full suite passes in CI