test: harden 82 Playwright specs for deterministic CI runs (#10967)

## Summary

Harden 98 E2E spec files and 8 fixtures/helpers for deterministic CI
runs by replacing race-prone patterns with retry-safe alternatives.

No source code changes -- only `browser_tests/` is touched.

## Changes

- **E2E spec hardening** (98 spec files, 6 fixtures, 2 helpers):

  | Fix class | Sites | Examples |
  |-----------|-------|---------:|
| `expect(await ...)` -> `expect.poll()` | ~153 | interaction,
defaultKeybindings, workflows, featureFlags |
| `const x = await loc.count(); expect(x)` -> `toHaveCount()` | ~19 |
menu, linkInteraction, assets, bottomPanelShortcuts |
| `nextFrame()` -> `waitForHidden()` after menu clicks | ~22 |
contextMenu, rightClickMenu, subgraphHelper |
| Redundant `nextFrame()` removed | many | defaultKeybindings, minimap,
builderSaveFlow |
| `expect(async () => { ... }).toPass()` retry blocks | 5 | interaction
(graphdialog dismiss guard) |
| `force:true` removed from `BaseDialog.close()` | 1 | BaseDialog
fixture |
| ContextMenu `waitForHidden` simplified (check-then-act race removed) |
1 | ContextMenu fixture |
| Non-deterministic node order -> proximity-based selection | 1 |
interaction (toggle dom widget) |
  | Tight poll timeout (250ms) -> >=2000ms | 2 | templates |

- **Helper improvements**: Exposed locator getters on
`ComfyPage.domWidgets`, `ToastHelper.toastErrors`, and
`WorkflowsSidebarTab.activeWorkflowLabel` so callers can use retrying
assertions (`toHaveCount()`, `toHaveText()`) directly.

- **Flake pattern catalog**: Added section 7 table to
`browser_tests/FLAKE_PREVENTION_RULES.md` documenting 8 pattern classes
for reviewers and future authors.

- **Docs**: Fixed bad examples in `browser_tests/README.md` to use
`expect.poll()`.

- **Breaking**: None
- **Dependencies**: None

## Review Focus

- All fixes follow the rules in
`browser_tests/FLAKE_PREVENTION_RULES.md`
- No behavioral changes to tests -- only timing/retry strategy is
updated
- The `ContextMenu.waitForHidden` simplification removes a
swallowed-error anti-pattern; both locators now use direct `waitFor({
state: 'hidden' })`

---------

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

browser_tests/tests/nodeHelp.spec.ts

@@ -433,8 +433,7 @@ This is documentation for a custom node.
       const imageCount = await images.count()
       for (let i = 0; i < imageCount; i++) {
         const img = images.nth(i)
-        const onError = await img.getAttribute('onerror')
-        expect(onError).toBeNull()
+        await expect(img).not.toHaveAttribute('onerror')
       }
 
       // Check that javascript: links are sanitized
@@ -442,10 +441,7 @@ This is documentation for a custom node.
       const linkCount = await links.count()
       for (let i = 0; i < linkCount; i++) {
         const link = links.nth(i)
-        const href = await link.getAttribute('href')
-        if (href !== null) {
-          expect(href).not.toContain('javascript:')
-        }
+        await expect(link).not.toHaveAttribute('href', /^javascript:/i)
       }
 
       // Safe content should remain
@@ -512,8 +508,7 @@ This is English documentation.
       await expect(helpPage.locator('.p-progressspinner')).not.toBeVisible()
 
       // Should show some content even on error
-      const content = await helpPage.textContent()
-      expect(content).toBeTruthy()
+      await expect(helpPage).not.toHaveText('')
     })
 
     test('Should update help content when switching between nodes', async ({