Improve session creation failure handling

Gabriel Maizorequested to merge
undead/xdg-desktop-portal-kde:fix/session-create-failure-handling into master

This MR improves session creation failure handling in the ScreenCast and RemoteDesktop portals.

  • Check WaylandIntegration::isStreamingAvailable() before creating sessions.
  • Call deleteLater() when a newly created session is invalid.
  • Log DBus registration failures in Session when registerObject() fails.

This keeps error handling clearer and avoids keeping invalid session objects around.

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