feat(appstore): use bounded stale cache when refresh fails

[joshtrichards]

• Resolves: #

Summary

This preserves access to (slightly stale) App Store metadata during temporary retrieval failures while preventing indefinitely stale or incompatible data from being used. This might also diminish need to use #55420 in some environments,

Before this change, a failed refresh returned an empty result even when a valid cached response was available. Now, we fall back to valid, same-version App Store cache data when refreshing the App Store fails, provided the cached data is no older than seven days:

Scenario	Before	After
Fresh, same-version cache	Return cached data	Return cached data
Stale cache, refresh succeeds	Return fresh data	Return fresh data
Stale cache, temporary refresh failure	Return []	Return same-version cached data, up to seven days old
Recent failure cooldown	Return []	Return same-version cached data, up to seven days old
Valid refresh with data: []	Return [] and do not refresh the cache	Store and return the valid empty result
Cache older than seven days and refresh fails	Return []	Return []
Cache from another Nextcloud version and refresh fails	Return []	Return []
Missing or unreadable cache and refresh fails	Return []	Return []

This improves resilience during temporary App Store or network failures without allowing indefinitely stale or incompatible metadata to be used.

Changes:

• Retain valid cached data for the current Nextcloud version while refreshing.
• Return stale cached data when refreshes fail or are suppressed by the recent-failure cooldown.
• Limit stale fallback data to seven days.
• Continue accepting valid empty responses as fresh data (to not overlook legitimate filtering changes).
• Reject missing, malformed, incompatible-version, and overly stale cache data.
• Add shared fetcher tests covering:
  • refresh failures with usable stale data;
  • the seven-day stale-data boundary;
  • the recent-failure cooldown;
  • valid empty refresh responses.
• Add child-fetcher regression coverage confirming that stale data still passes through:
  • AppFetcher allowlist filtering;
  • AppDiscoverFetcher expiry filtering.

User impact

Temporary App Store or network failures no longer cause App Store browsing and metadata consumers to immediately receive an empty result when usable cached metadata is available.

For example, if the App Store endpoint is temporarily unavailable while an instance has a six-day-old cache for its current Nextcloud version, users can continue browsing the cached App Store metadata. If the cache is more than seven days old, or belongs to another Nextcloud version, the fetcher still returns an empty result rather than presenting potentially misleading data.

Testing

• PHPUnit coverage added for the base fetcher fallback behavior.
• Regression coverage added for AppFetcher and AppDiscoverFetcher.

TODO

• Backport? (Kinda a bug fix / kinda an enhancement... so...)

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI