fix: accessibility tree in isolated sessions on Fedora

[davidselassie]

Summary

Fix accessibility_tree (and all AT-SPI tools) returning empty results or crashing in isolated virtual sessions on Fedora. Extract AT-SPI binary paths from D-Bus service files instead of hardcoding distro-specific paths, and explicitly bootstrap at-spi2-registryd in isolated sessions where no session manager is present to auto-activate it.

Motivation

1. Wrong binary path: at-spi-bus-launcher was hardcoded to /usr/lib/at-spi-bus-launcher, but Fedora installs it at /usr/libexec/at-spi-bus-launcher. The launcher silently failed to start, so the AT-SPI bus was never created. AT-SPI queries crashed with "Couldn't connect to accessibility bus".

   Fix: Extract the binary path from the D-Bus service file (/usr/share/dbus-1/services/org.a11y.Bus.service) using sed. This is more robust than hardcoding paths for each distro because the service file location is standardized by the D-Bus spec and always contains the correct Exec= path for the current system — whether that's /usr/libexec (Fedora), /usr/lib (Arch), /usr/lib/at-spi2-core (Debian), or anything else. D-Bus auto-activation was also considered but doesn't work from dbus-run-session (fails with "Permission denied").

2. Missing registryd: Even with the bus launcher running, at-spi2-registryd was never started. D-Bus auto-activation fails because the .service file uses --use-gnome-session, which requires a session manager that doesn't exist in isolated sessions. Without the registryd, no applications register their accessibility trees, so accessibility_tree returned "(no accessible applications found)".

   Fix: Extract the registryd binary path from its service file (/usr/share/dbus-1/accessibility-services/org.a11y.atspi.Registry.service) the same way, then start it directly without --use-gnome-session. The wrapper script also sets IsEnabled=true on the AT-SPI bus so that apps know to register their accessibility trees. No explicit cleanup is needed — session.stop() already kills the entire process group via os.killpg(), which covers the registryd.

How to Test

1. Verify path extraction resolves to a real binary on your system:

   sed -n 's/^Exec=\([^ ]*\).*/\1/p' /usr/share/dbus-1/services/org.a11y.Bus.service
   

2. Smoke test — should print an accessibility tree with elements, not "(no accessible applications found)":

   printf 'session_start app_command=kcalc\naccessibility_tree\nsession_stop\nquit\n' \
     | uv run kwin-mcp-cli
   

Checklist

• uv run ruff check . passes
• uv run ruff format --check . passes
• uv run ty check passes
• CHANGELOG.md updated (if user-facing change)
• README.md updated (if new tools or changed behavior) — N/A, no new tools or changed behavior

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