test(bidi): wait for dialog to be dismissed in beforeunload test

[hbenl]

This test throws dialog.dismiss: Test ended. with Firefox/Bidi on Linux (and the failure gets misattributed to another test in CI and causes other tests to be skipped).
Unlike previous attempts, this PR doesn't swallow any exceptions but waits for the dialog to be dismissed before the test ends.

[github-actions]

Test results for "tests 1"

9 failed
❌ [default] › run-tests.spec.ts:226 › should show error message @vscode-extension
❌ [default] › run-tests.spec.ts:278 › should show soft error messages @vscode-extension
❌ [default] › run-tests.spec.ts:818 › should report project-specific failures @vscode-extension
❌ [default-reuse] › run-tests.spec.ts:226 › should show error message @vscode-extension
❌ [default-reuse] › run-tests.spec.ts:278 › should show soft error messages @vscode-extension
❌ [default-reuse] › run-tests.spec.ts:818 › should report project-specific failures @vscode-extension
❌ [default-trace] › run-tests.spec.ts:226 › should show error message @vscode-extension
❌ [default-trace] › run-tests.spec.ts:278 › should show soft error messages @vscode-extension
❌ [default-trace] › run-tests.spec.ts:818 › should report project-specific failures @vscode-extension

7 flaky

⚠️ [chromium-library] › library/beforeunload.spec.ts:130 › should support dismissing the dialog multiple times `@ubuntu-22.04-chromium-tip-of-tree`
⚠️ [chromium-library] › library/popup.spec.ts:260 › 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/trace-viewer.spec.ts:1223 › should display language-specific locators `@chromium-ubuntu-22.04-node24`
⚠️ [chromium-library] › library/trace-viewer.spec.ts:1223 › should display language-specific locators `@chromium-ubuntu-22.04-node22`
⚠️ [firefox-library] › library/inspector/cli-codegen-1.spec.ts:1080 › cli codegen › should not throw csp directive violation errors `@firefox-ubuntu-22.04-node20`

38761 passed, 846 skipped

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Merge workflow run.

[github-actions]

Test results for "MCP"

5340 passed, 172 skipped

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Merge workflow run.