test: remove unused select2Nodes and fix flaky timeout

- Remove select2Nodes helper method from NodeOperationsHelper
- Replace usages with selectNodes(['CLIP Text Encode (Prompt)'])
- Increase flaky timeout in mobileBaseline test from 256ms to 5000ms

Amp-Thread-ID: https://ampcode.com/threads/T-019c15b9-efed-72eb-b092-1e92a11f44e3
Co-authored-by: Amp <amp@ampcode.com>

browser_tests/tests/mobileBaseline.spec.ts

@@ -10,7 +10,7 @@ test.describe(
       await comfyPage.executeCommand('Comfy.ClearWorkflow')
       await expect(async () => {
         expect(await comfyPage.nodeOps.getGraphNodesCount()).toBe(0)
-      }).toPass({ timeout: 256 })
+      }).toPass({ timeout: 5000 })
       await comfyPage.nextFrame()
       await expect(comfyPage.canvas).toHaveScreenshot('mobile-empty-canvas.png')
     })