feat(ydotool): add comprehensive unit tests for ydotool injector

Rebased from Jules draft #273.
- Add TestHarness for controlled test environment
- Add tests for socket discovery, binary permissions, text injection
- Gate tests behind ydotool feature flag

Author: Coldaine

crates/coldvox-text-injection/src/tests/mod.rs

@@ -3,6 +3,7 @@
 pub mod real_injection;
 #[cfg(feature = "real-injection-tests")]
 pub mod test_harness;
+#[cfg(all(unix, feature = "ydotool"))]
 pub mod test_ydotool_injector;
 pub mod test_utils;
 pub mod wl_copy_basic_test;