fix(unix): Preserve AT-SPI cache signal structure arguments

[luccahuguet]

The Unix adapter flattens the structure argument in Cache.AddAccessible and Cache.RemoveAccessible. GNOME AT-SPI rejects the resulting signatures, leaving clients with stale cache state that can disrupt Orca focus and selection reporting.

Wrap the body in a one-element tuple in the shared emit_cache_signal helper, and require Type for tuple serialization. Both signals then preserve their required structure:

Signal	Before	After
AddAccessible	(so)(so)(so)iiassusau	((so)(so)(so)iiassusau)
RemoveAccessible	so	(so)

Only two production lines in adapters/unix/src/atspi/bus.rs differ from upstream 466e24e252103f4d82bd6fbab98141b551e46e0c. The standalone private-D-Bus test was removed at the maintainer's request pending a proper Unix test harness. No manifests, dependencies, lockfile, or generated changelogs change.

Validation on the revised candidate, x86_64 Linux:

• cargo +1.87.0 test --locked -p accesskit_unix: 5 passed.
• cargo fmt --all -- --check: passed.
• git diff --check: passed.

Before test removal, the private D-Bus regression failed without the fix and passed with it, checking the encoded SIGNATURE headers and round-tripping both payloads. Removing the test does not change the verified production code. GNOME's AddAccessible handler and RemoveAccessible handler require the corrected signatures.

The same correction was separately verified downstream with Orca 49.4 and AT-SPI 2.58.1 on native Wayland using this backport. That run used accesskit_unix 0.22.1 / zbus 5.18.0 / zvariant 5.13.1. Current-upstream native Orca and non-Linux runtime checks were not run.

Closes #818.

[DataTriny]

Hello @luccahuguet,

Thanks for the fix. As they stand, the tests are unfortunately not very useful. I would remove them until we have a proper test harness for Unix.

[luccahuguet]

Hello @luccahuguet,

Thanks for the fix. As they stand, the tests are unfortunately not very useful. I would remove them until we have a proper test harness for Unix.

thanks! I'll remove them

[luccahuguet]

(done) @DataTriny

[DataTriny]

Thanks!