refactor(tests): use DefaultGraphPositions constants for raw coordinates

- Replace hardcoded coordinates with DefaultGraphPositions references

- Fix y-coordinate inconsistency (625 vs 645) in emptyLatentWidgetClick usage

Amp-Thread-ID: https://ampcode.com/threads/T-019c161a-ed63-7545-bbd0-655efec143c4
Co-authored-by: Amp <amp@ampcode.com>

browser_tests/fixtures/ComfyPage.ts

@@ -591,7 +591,10 @@ export class ComfyPage {
   }
 
   async dragNode2() {
-    await this.canvasOps.dragAndDrop({ x: 622, y: 400 }, { x: 622, y: 300 })
+    await this.canvasOps.dragAndDrop(DefaultGraphPositions.textEncodeNode2, {
+      x: DefaultGraphPositions.textEncodeNode2.x,
+      y: 300
+    })
     await this.nextFrame()
   }