fix: stabilize flaky screenshot tests that rely on search box timings (#9426)

## Summary

Fixes flaky screenshot test that was identified during the PR #9400
snapshot update, where baselines regenerated without any code changes
affecting them.

**Root cause:** `fillAndSelectFirstNode('KSampler')` selected `nth(0)`
from the autocomplete dropdown, but search result ordering is
non-deterministic when the search box opens via link release with filter
chips. Sometimes 'Preview Image' appeared as the first result instead of
'KSampler', causing a completely different node to be added.

**Fix:** Added an `exact: true` option to `fillAndSelectFirstNode` that
uses an `aria-label` selector to click the specific matching result
instead of blindly selecting the first item.

- Fixes #4658

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9426-fix-stabilize-flaky-screenshot-tests-for-search-results-and-image-preview-31a6d73d365081598167ce285416995c)
by [Unito](https://www.unito.io)

---------

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

browser_tests/tests/nodeSearchBox.spec.ts

@@ -110,7 +110,9 @@ test.describe('Node search box', { tag: '@node' }, () => {
       await comfyPage.canvasOps.disconnectEdge()
       await expect(comfyPage.searchBox.input).toHaveCount(1)
       await comfyPage.page.locator('.p-chip-remove-icon').click()
-      await comfyPage.searchBox.fillAndSelectFirstNode('KSampler')
+      await comfyPage.searchBox.fillAndSelectFirstNode('KSampler', {
+        exact: true
+      })
       await expect(comfyPage.canvas).toHaveScreenshot(
         'added-node-no-connection.png'
       )