image_viewer: Cover pinch zoom in the PDF interaction test

Assert that a pinch gesture scales the PDF zoom by the gesture delta,
pinning the on_pinch wiring across real event dispatch.

crates/image_viewer/src/image_viewer.rs

@@ -5840,6 +5840,22 @@ mod tests {
             offset_h.x >= -cx.read(|cx| image_view.read(cx).pdf_scroll_handle.max_offset().x),
             "horizontal pan should stay within the scrollable range"
         );
+
+        // Trackpad pinch zooms around the touch point by the gesture delta.
+        let zoom_before_pinch = cx.read(|cx| image_view.read(cx).zoom_level);
+        cx.simulate_event(PinchEvent {
+            position: anchor,
+            delta: 0.5,
+            modifiers: Modifiers::default(),
+            phase: TouchPhase::Moved,
+        });
+        cx.run_until_parked();
+
+        let zoom_after_pinch = cx.read(|cx| image_view.read(cx).zoom_level);
+        assert!(
+            (zoom_after_pinch - zoom_before_pinch * 1.5).abs() < 1e-3,
+            "pinch delta 0.5 should scale zoom by 1.5x, got {zoom_before_pinch} -> {zoom_after_pinch}"
+        );
     }
 
     #[gpui::test]