Bug 515974

SUMMARY
Discover retries indefinitely on 404'd Flathub delta-index instead of falling back to full metadata fetch

STEPS TO REPRODUCE
1. Have Flathub configured as a Flatpak remote
2. Open Discover when a previously-referenced delta-index file has been removed from Flathub's CDN (returns HTTP 404)
3. Observe Discover repeatedly attempting to fetch the URL and reporting timeout errors

OBSERVED RESULT
Discover logs repeated "libostree HTTP error from remote flathub for <https://dl.flathub.org/repo/delta-indexes/...>: Timeout was reached" messages. It never falls back to a full metadata fetch, effectively making Discover unable to load Flatpak application listings. The delta-index URL returns HTTP 404 when checked directly with curl.

EXPECTED RESULT
Discover should detect that the delta-index returns 404 (not a transient network error) and fall back to a full appstream metadata fetch, as the flatpak CLI does successfully in the same situation.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma:
KDE Plasma Version: [run kinfo]
KDE Frameworks Version: [run kinfo]
Qt Version: [run kinfo]
Fedora 43 (KDE Spin)

ADDITIONAL INFORMATION
The flatpak CLI handles this correctly  "flatpak update --appstream" forces a full metadata refresh and resolves the issue. Discover's libostree integration appears to treat HTTP 404 responses the same as transient timeout errors, leading to an infinite retry loop.

Relevant log lines from journalctl:
libostree HTTP error from remote flathub for <https://dl.flathub.org/repo/delta-indexes/7Q/dJHeoulSA7bChbqpRu7BfixR8VspcMXi_P2BvYkWY.index>: Timeout was reached

curl -I on the same URL returns HTTP 404.
Workaround: "flatpak repair --user && flatpak update --appstream" then relaunch Discover.