test: add E2E tests for Load3D model upload and drag-drop and basic e2e for 3d viewer (#10957)

## Summary
Add tests verifying real model loading:
- Upload cube.obj via file chooser button
- Drag-and-drop cube.obj onto the 3D canvas
- Add data-testid to LoadingOverlay for stable test selectors.
Add tests verifying 3d viewer openning:
- Open viewer from Load3D node via expand button, verify canvas and
controls sidebar
- Cancel button closes the viewer dialog

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10957-test-add-E2E-tests-for-Load3D-model-upload-and-drag-drop-and-basic-e2e-for-3d-viewer-33c6d73d3650810c8ff8ed656a5164a6)
by [Unito](https://www.unito.io)

---------

Co-authored-by: github-actions <github-actions@github.com>

browser_tests/tests/load3d/Load3DHelper.ts

@@ -1,5 +1,8 @@
+import { expect } from '@playwright/test'
 import type { Locator } from '@playwright/test'
 
+import { TestIds } from '@e2e/fixtures/selectors'
+
 export class Load3DHelper {
   constructor(readonly node: Locator) {}
 
@@ -19,6 +22,10 @@ export class Load3DHelper {
     return this.node.locator('input[type="color"]')
   }
 
+  get openViewerButton(): Locator {
+    return this.node.getByRole('button', { name: /open in 3d viewer/i })
+  }
+
   getUploadButton(label: string): Locator {
     return this.node.getByText(label)
   }
@@ -37,4 +44,10 @@ export class Load3DHelper {
       el.dispatchEvent(new Event('input', { bubbles: true }))
     }, hex)
   }
+
+  async waitForModelLoaded(): Promise<void> {
+    await expect(this.node.getByTestId(TestIds.loading.overlay)).toBeHidden({
+      timeout: 30000
+    })
+  }
 }