chore: swallow TargetClosedErrors in beforeunload dialog.dismiss()

[hbenl]

See #39701 (comment). This swallows TargetClosedErrors only for beforeunload dialogs because this test checks that dialog.dismiss() will throw for alert dialogs.

[github-actions]

Test results for "MCP"

7 failed
❌ [chrome] › mcp/test-debug.spec.ts:167 › test_debug (pause/snapshot/resume) @mcp-windows-latest
❌ [chromium] › mcp/test-run.spec.ts:87 › test_run filters @mcp-windows-latest
❌ [chromium] › mcp/test-setup.spec.ts:60 › setup should run global setup and teardown @mcp-windows-latest
❌ [chrome] › mcp/test-setup.spec.ts:60 › setup should run global setup and teardown @mcp-macos-latest
❌ [chromium] › mcp/cli-help.spec.ts:24 › prints command help @mcp-macos-latest
❌ [msedge] › mcp/test-debug.spec.ts:21 › test_debug (passed) @mcp-windows-latest
❌ [msedge] › mcp/test-debug.spec.ts:48 › test_debug (pause/resume) @mcp-windows-latest

5451 passed, 340 skipped

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Merge workflow run.

[github-actions]

Test results for "tests 1"

2 failed
❌ [playwright-test] › reporter.spec.ts:251 › created › should not have internal error when steps are finished after timeout @macos-latest-node20
❌ [playwright-test] › ui-mode-test-network-tab.spec.ts:397 › should not preserve selection across test runs @windows-latest-node20

8 flaky

⚠️ [chromium-library] › library/popup.spec.ts:261 › should not throw when click closes popup `@ubuntu-22.04-chromium-tip-of-tree`
⚠️ [chromium-library] › library/trace-viewer.spec.ts:1223 › should display language-specific locators `@chromium-ubuntu-22.04-arm-node20`
⚠️ [chromium-library] › library/video.spec.ts:358 › screencast › should capture navigation `@chromium-ubuntu-22.04-arm-node20`
⚠️ [chromium-library] › library/popup.spec.ts:261 › should not throw when click closes popup `@chromium-ubuntu-22.04-node20`
⚠️ [chromium-library] › library/trace-viewer.spec.ts:1223 › should display language-specific locators `@chromium-ubuntu-22.04-node22`
⚠️ [webkit-library] › library/screencast.spec.ts:75 › start reuses existing screencast when video recording is running `@webkit-ubuntu-22.04-node20`
⚠️ [playwright-test] › ui-mode-test-output.spec.ts:118 › should collapse repeated console messages for test `@macos-latest-node20`
⚠️ [playwright-test] › ui-mode-test-source.spec.ts:129 › should show syntax errors in file `@windows-latest-node20`

38804 passed, 845 skipped

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Merge workflow run.

[pavelfeldman]

Why the beforeunload?