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/fixtures/components/ComfyNodeSearchBox.ts

@@ -55,15 +55,22 @@ export class ComfyNodeSearchBox {
 
   async fillAndSelectFirstNode(
     nodeName: string,
-    options?: { suggestionIndex: number }
+    options?: { suggestionIndex?: number; exact?: boolean }
   ) {
     await this.input.waitFor({ state: 'visible' })
     await this.input.fill(nodeName)
     await this.dropdown.waitFor({ state: 'visible' })
-    await this.dropdown
-      .locator('li')
-      .nth(options?.suggestionIndex || 0)
-      .click()
+    if (options?.exact) {
+      await this.dropdown
+        .locator(`li[aria-label="${nodeName}"]`)
+        .first()
+        .click()
+    } else {
+      await this.dropdown
+        .locator('li')
+        .nth(options?.suggestionIndex || 0)
+        .click()
+    }
   }
 
   async addFilter(filterValue: string, filterType: string) {