test(bidi): fix 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).
Unlike #39331, this PR only swallows the exception for Firefox/Bidi, so that any unexpected exceptions for other browsers/protocols will still cause the test to fail.

[github-actions]

Test results for "tests 1"

6 failed
❌ [playwright-test] › aria-snapshot-file.spec.ts:180 › should respect timeout @macos-latest-node20
❌ [playwright-test] › playwright.unhandled.spec.ts:67 › should produce unhandledRejection when context.route raises @macos-latest-node20
❌ [playwright-test] › test-step.spec.ts:1053 › web assertion inside expect.poll @macos-latest-node20
❌ [playwright-test] › test-step.spec.ts:1106 › should report expect steps @macos-latest-node20
❌ [playwright-test] › to-have-screenshot.spec.ts:301 › should successfully screenshot a page with infinite animation with disableAnimation: true @macos-latest-node20
❌ [playwright-test] › ui-mode-test-network-tab.spec.ts:371 › should not preserve selection across test runs @macos-latest-node20

3 flaky

⚠️ [chromium-page] › page/page-request-continue.spec.ts:754 › propagate headers cross origin redirect after interception `@ubuntu-22.04-chromium-tip-of-tree`
⚠️ [webkit-library] › library/browsercontext-add-cookies.spec.ts:426 › should allow unnamed cookies `@webkit-ubuntu-22.04-node20`
⚠️ [playwright-test] › ui-mode-trace.spec.ts:700 › should indicate current test status `@macos-latest-node20`

38563 passed, 843 skipped

---

Merge workflow run.

[github-actions]

Test results for "MCP"

3 failed
❌ [chromium] › mcp/tracing.spec.ts:21 › check that trace is saved with --save-trace @mcp-windows-latest
❌ [chrome] › mcp/http.spec.ts:263 › http transport shared context @mcp-macos-15
❌ [chrome] › mcp/sse.spec.ts:233 › sse transport shared context @mcp-macos-15

4893 passed, 135 skipped

---

Merge workflow run.

[yury-s]

I don't see how this helps, the users don't expect dialog.dismiss() to throw in regular scenarios. Let's just keep the test marked in the bidi expectations as failing for now.