fix(interfaces): allow the desktop interface pinging the IBus portal

IBus sends org.freedesktop.DBus.Peer.Ping before completing a portal
connection:

    g_dbus_connection_call (
            bus->priv->connection,
            IBUS_SERVICE_PORTAL,
            IBUS_PATH_IBUS,
            "org.freedesktop.DBus.Peer",
            "Ping",
            ...
            _bus_connect_start_portal_cb,
            g_object_ref (bus));

The desktop interface currently denies that call, preventing the input
context from becoming ready and causing key events to queue and eventually
be dropped.

Allow Ping specifically for org.freedesktop.portal.IBus and verify the
permission in the desktop interface AppArmor test.

Assisted-by: Codex:gpt-5.6-sol context7
Signed-off-by: 林博仁(Buo-ren Lin) <buo.ren.lin@gmail.com>

Author: brlin-tw

interfaces/builtin/desktop.go

@@ -442,6 +442,14 @@ dbus (send)
       member=CreateInputContext
       peer=(name=org.freedesktop.portal.IBus),
 
+# Allow IBus to verify that the portal is responsive before connecting
+dbus (send)
+      bus=session
+      path=/org/freedesktop/IBus
+      interface=org.freedesktop.DBus.Peer
+      member=Ping
+      peer=(name=org.freedesktop.portal.IBus),
+
 dbus (send, receive)
       bus=session
       path=/org/freedesktop/IBus/InputContext_[0-9]*

interfaces/builtin/desktop_test.go

@@ -146,6 +146,7 @@ func (s *DesktopInterfaceSuite) TestAppArmorSpec(c *C) {
 	c.Check(spec.SnippetForTag("snap.consumer.app"), testutil.Contains, "owner @{HOME}/.config/gtk-3.0/{*.css,**/*.css} r,")
 	c.Check(spec.SnippetForTag("snap.consumer.app"), testutil.Contains, "owner @{HOME}/.config/gtk-4.0/{*.css,**/*.css} r,")
 	c.Check(spec.SnippetForTag("snap.consumer.app"), testutil.Contains, "# Allow access to xdg-desktop-portal and xdg-document-portal")
+	c.Check(spec.SnippetForTag("snap.consumer.app"), testutil.Contains, "interface=org.freedesktop.DBus.Peer\n      member=Ping\n      peer=(name=org.freedesktop.portal.IBus),")
 
 	// check desktop interface uses correct label for Mutter when provided
 	// by the system