Fix VisualDensity's vertical lerp implementation (#96597)

Fixes #96591

packages/flutter/lib/src/material/theme_data.dart

@@ -2592,7 +2592,7 @@ class VisualDensity with Diagnosticable {
   static VisualDensity lerp(VisualDensity a, VisualDensity b, double t) {
     return VisualDensity(
       horizontal: lerpDouble(a.horizontal, b.horizontal, t)!,
-      vertical: lerpDouble(a.horizontal, b.horizontal, t)!,
+      vertical: lerpDouble(a.vertical, b.vertical, t)!,
     );
   }
 

packages/flutter/test/material/text_theme_test.dart

@@ -220,4 +220,21 @@ void main() {
     expect(lerped.labelMedium, null);
     expect(lerped.labelSmall, null);
   });
+
+  test('VisualDensity.lerp', () {
+    const VisualDensity a = VisualDensity(horizontal: 1.0, vertical: .5);
+    const VisualDensity b = VisualDensity(horizontal: 2.0, vertical: 1.0);
+
+    final VisualDensity noLerp = VisualDensity.lerp(a, b, 0.0);
+    expect(noLerp.horizontal, 1.0);
+    expect(noLerp.vertical, .5);
+
+    final VisualDensity quarterLerp = VisualDensity.lerp(a, b, .25);
+    expect(quarterLerp.horizontal, 1.25);
+    expect(quarterLerp.vertical, .625);
+
+    final VisualDensity fullLerp = VisualDensity.lerp(a, b, 1.0);
+    expect(fullLerp.horizontal, 2.0);
+    expect(fullLerp.vertical, 1.0);
+  });
 }