Make Rust-only backend test target explicit

The backend-tests crate is intentionally an integration-test harness, but declaring the target explicitly makes Cargo discovery and review intent unambiguous. The harness comment also records that path-included modules run their own cfg(test) unit tests for hotkey, recorder, and insertion without linking the full Tauri library.\n\nConstraint: Stay within Rust-only backend tests; do not add a Tauri lib/runtime target.\nRejected: Add src/lib.rs just to satisfy target discovery | would introduce an unused crate surface instead of the actual test harness.\nConfidence: high\nScope-risk: narrow\nTested: cargo test --manifest-path openless-all/app/src-tauri/backend-tests/Cargo.toml -- --list | grep -E '^(hotkey|recorder|insertion)::'\nTested: cargo test --manifest-path openless-all/app/src-tauri/backend-tests/Cargo.toml\nTested: git diff --check\nNot-tested: GitHub Windows runner rerun result pending after push.\n\nCo-authored-by: OmX <omx@oh-my-codex.dev>

openless-all/app/src-tauri/backend-tests/Cargo.toml

@@ -5,6 +5,10 @@ edition = "2021"
 rust-version = "1.77"
 publish = false
 
+[[test]]
+name = "backend_rust"
+path = "tests/backend_rust.rs"
+
 [dependencies]
 arboard = "3"
 cpal = "0.15"

openless-all/app/src-tauri/backend-tests/tests/backend_rust.rs

@@ -2,6 +2,8 @@
 //!
 //! 这个测试 crate 只把纯 Rust 后端模块按源码路径编进来，不链接完整 Tauri
 //! `openless_lib`，避免 Windows CI 在 test harness 启动前被桌面运行时 DLL 拦截。
+//! Cargo 以 `cfg(test)` 编译这些 path-included 模块，所以各模块自己的
+//! `#[cfg(test)]` 单测会在这里实际执行（见 hotkey / recorder / insertion）。
 
 #![allow(dead_code, unused_variables)]