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/memory/traits.rs

@@ -66,3 +66,53 @@ pub trait Memory: Send + Sync {
     /// Health check
     async fn health_check(&self) -> bool;
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn memory_category_display_outputs_expected_values() {
+        assert_eq!(MemoryCategory::Core.to_string(), "core");
+        assert_eq!(MemoryCategory::Daily.to_string(), "daily");
+        assert_eq!(MemoryCategory::Conversation.to_string(), "conversation");
+        assert_eq!(
+            MemoryCategory::Custom("project_notes".into()).to_string(),
+            "project_notes"
+        );
+    }
+
+    #[test]
+    fn memory_category_serde_uses_snake_case() {
+        let core = serde_json::to_string(&MemoryCategory::Core).unwrap();
+        let daily = serde_json::to_string(&MemoryCategory::Daily).unwrap();
+        let conversation = serde_json::to_string(&MemoryCategory::Conversation).unwrap();
+
+        assert_eq!(core, "\"core\"");
+        assert_eq!(daily, "\"daily\"");
+        assert_eq!(conversation, "\"conversation\"");
+    }
+
+    #[test]
+    fn memory_entry_roundtrip_preserves_optional_fields() {
+        let entry = MemoryEntry {
+            id: "id-1".into(),
+            key: "favorite_language".into(),
+            content: "Rust".into(),
+            category: MemoryCategory::Core,
+            timestamp: "2026-02-16T00:00:00Z".into(),
+            session_id: Some("session-abc".into()),
+            score: Some(0.98),
+        };
+
+        let json = serde_json::to_string(&entry).unwrap();
+        let parsed: MemoryEntry = serde_json::from_str(&json).unwrap();
+
+        assert_eq!(parsed.id, "id-1");
+        assert_eq!(parsed.key, "favorite_language");
+        assert_eq!(parsed.content, "Rust");
+        assert_eq!(parsed.category, MemoryCategory::Core);
+        assert_eq!(parsed.session_id.as_deref(), Some("session-abc"));
+        assert_eq!(parsed.score, Some(0.98));
+    }
+}