add isDisposed getter to DatabaseTracker for external state checking

Author: suojae

drift/lib/src/sqlite3/database_tracker.dart

@@ -27,6 +27,9 @@ class DatabaseTracker {
   /// Whether this [DatabaseTracker] has been disposed.
   bool _isDisposed = false;
 
+  /// Public getter to check if this tracker has been disposed.
+  bool get isDisposed => _isDisposed;
+
   /// Creates a new tracker with necessary tables.
   DatabaseTracker()
       : _db = sqlite3.open(

drift/test/database/database_tracker_test.dart

@@ -61,5 +61,12 @@ void main() {
       expect(() => tracker.dispose(), returnsNormally);
       expect(() => tracker.dispose(), returnsNormally);
     });
+
+    test('isDisposed reflects tracker state', () {
+      expect(tracker.isDisposed, isFalse);
+
+      tracker.dispose();
+      expect(tracker.isDisposed, isTrue);
+    });
   });
 }