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/observability/traits.rs

@@ -58,3 +58,72 @@ pub trait Observer: Send + Sync + 'static {
         self
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::sync::Mutex;
+    use std::time::Duration;
+
+    #[derive(Default)]
+    struct DummyObserver {
+        events: Mutex<u64>,
+        metrics: Mutex<u64>,
+    }
+
+    impl Observer for DummyObserver {
+        fn record_event(&self, _event: &ObserverEvent) {
+            let mut guard = self.events.lock().unwrap();
+            *guard += 1;
+        }
+
+        fn record_metric(&self, _metric: &ObserverMetric) {
+            let mut guard = self.metrics.lock().unwrap();
+            *guard += 1;
+        }
+
+        fn name(&self) -> &str {
+            "dummy-observer"
+        }
+    }
+
+    #[test]
+    fn observer_records_events_and_metrics() {
+        let observer = DummyObserver::default();
+
+        observer.record_event(&ObserverEvent::HeartbeatTick);
+        observer.record_event(&ObserverEvent::Error {
+            component: "test".into(),
+            message: "boom".into(),
+        });
+        observer.record_metric(&ObserverMetric::TokensUsed(42));
+
+        assert_eq!(*observer.events.lock().unwrap(), 2);
+        assert_eq!(*observer.metrics.lock().unwrap(), 1);
+    }
+
+    #[test]
+    fn observer_default_flush_and_as_any_work() {
+        let observer = DummyObserver::default();
+
+        observer.flush();
+        assert_eq!(observer.name(), "dummy-observer");
+        assert!(observer.as_any().downcast_ref::<DummyObserver>().is_some());
+    }
+
+    #[test]
+    fn observer_event_and_metric_are_cloneable() {
+        let event = ObserverEvent::ToolCall {
+            tool: "shell".into(),
+            duration: Duration::from_millis(10),
+            success: true,
+        };
+        let metric = ObserverMetric::RequestLatency(Duration::from_millis(8));
+
+        let cloned_event = event.clone();
+        let cloned_metric = metric.clone();
+
+        assert!(matches!(cloned_event, ObserverEvent::ToolCall { .. }));
+        assert!(matches!(cloned_metric, ObserverMetric::RequestLatency(_)));
+    }
+}