test: deepen and complete project-wide test coverage (#297)

* test: deepen coverage for health doctor provider and tunnels

* test: add broad trait and module re-export coverage

src/heartbeat/mod.rs

@@ -1 +1,34 @@
 pub mod engine;
+
+#[cfg(test)]
+mod tests {
+    use crate::config::HeartbeatConfig;
+    use crate::heartbeat::engine::HeartbeatEngine;
+    use crate::observability::NoopObserver;
+    use std::sync::Arc;
+
+    #[test]
+    fn heartbeat_engine_is_constructible_via_module_export() {
+        let temp = tempfile::tempdir().unwrap();
+        let engine = HeartbeatEngine::new(
+            HeartbeatConfig::default(),
+            temp.path().to_path_buf(),
+            Arc::new(NoopObserver),
+        );
+
+        let _ = engine;
+    }
+
+    #[tokio::test]
+    async fn ensure_heartbeat_file_creates_expected_file() {
+        let temp = tempfile::tempdir().unwrap();
+        let workspace = temp.path();
+
+        HeartbeatEngine::ensure_heartbeat_file(workspace)
+            .await
+            .unwrap();
+
+        let heartbeat_path = workspace.join("HEARTBEAT.md");
+        assert!(heartbeat_path.exists());
+    }
+}