Bug#1148931: XbSilo "Unsupported query" failure

[Ovi Cazacu]

Package: gnome-software
Version: 48.3-2
Severity: important

-- Description --

After a fresh Flathub remote add (`flatpak remote-add --if-not-exists
flathub https://flathub.org/repo/flathub.flatpakrepo`, system-wide
scope), GNOME Software shows zero apps in every view (curated,
recent, search, category browsing), despite the Flatpak stack itself
being fully healthy.

Confirmed via `flatpak remote-info flathub org.gimp.GIMP` that
appstream data is present and valid, and confirmed the on-disk
appstream commit at
/var/lib/flatpak/appstream/flathub/x86_64/active/ contains a full
49MB appstream.xml plus icons/.

Running `gnome-software --verbose 2>&1 | grep -i query` shows every
single plugin — not just flatpak — failing identically:

    Gs  plugin 'appstream' failed to list apps: Unsupported query
    Gs  plugin 'flatpak' failed to list apps: Unsupported query
    Gs  plugin 'fwupd' failed to list apps: Unsupported query
    Gs  plugin 'packagekit' failed to list apps: Unsupported query

This happens even after the .xmlb silo caches
(~/.cache/gnome-software/appstream/components.xmlb and
~/.cache/gnome-software/flatpak-user-user/components.xmlb) are
confirmed present on disk and loading successfully (no "file not
found" errors on second launch) — ruling out a cache/race condition.

Since this affects every plugin uniformly, including ones with no
relation to flatpak/appstream data (fwupd, packagekit), this appears
to be an incompatibility between the XPath-style queries GNOME
Software 48.3 issues against XbSilo and the libxmlb version
currently in Trixie.

-- System Information --
gnome-software: 48.3-2
libxmlb2: 0.3.22-1
gnome-software-plugin-flatpak: 48.3-2
flatpak: 1.16.1
Distribution: Debian Trixie (13)
Architecture: amd64

-- Steps to reproduce --
1. Fresh Debian Trixie install with GNOME
2. sudo apt install flatpak gnome-software-plugin-flatpak
3. flatpak remote-add --if-not-exists flathub
   https://flathub.org/repo/flathub.flatpakrepo
4. sudo flatpak update --appstream
5. Launch gnome-software --verbose
6. Observe: no apps shown anywhere in the UI; verbose log shows
   "Unsupported query" for every enabled plugin
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