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/vueNodes/nodeStates/pin.spec.ts

@@ -61,10 +61,9 @@ test.describe('Vue Node Pin', () => {
     )
 
     // Verify the node is not dragged (same position before and after click-and-drag)
-    const headerPosAfterDrag = await checkpointNodeHeader.boundingBox()
-    if (!headerPosAfterDrag)
-      throw new Error('Failed to get header position after drag')
-    expect(headerPosAfterDrag).toEqual(headerPos)
+    await expect
+      .poll(async () => await checkpointNodeHeader.boundingBox())
+      .toEqual(headerPos)
 
     // Unpin the node with the hotkey
     await checkpointNodeHeader.click()
@@ -77,9 +76,8 @@ test.describe('Vue Node Pin', () => {
     )
 
     // Verify the node is dragged
-    const headerPosAfterDrag2 = await checkpointNodeHeader.boundingBox()
-    if (!headerPosAfterDrag2)
-      throw new Error('Failed to get header position after drag')
-    expect(headerPosAfterDrag2).not.toEqual(headerPos)
+    await expect
+      .poll(async () => await checkpointNodeHeader.boundingBox())
+      .not.toEqual(headerPos)
   })
 })