fix(runner): force-kill worker that hangs on stop

[dgozman]

Summary

• Watchdog around __stop__: if the worker process does not exit within 5 min (overridable via PWTEST_CHILD_PROCESS_TIMEOUT for tests), it is force-killed (taskkill /T /F on Windows, SIGKILL elsewhere).
• Force-kill emits a new processError event that the dispatcher reports via reporter.onError, so a hung worker fails the run.

Fixes #39753

[github-actions]

Test results for "MCP"

12 failed
❌ [chromium] › mcp/dashboard.spec.ts:185 › should start dashboard and annotate when no dashboard is running @mcp-windows-latest-chromium
❌ [chromium] › mcp/dashboard.spec.ts:207 › should enter annotate mode on fresh dashboard.tsx mount with -s --annotate @mcp-windows-latest-chromium
❌ [chromium] › mcp/dashboard.spec.ts:231 › should annotate via direct browser_annotate MCP call @mcp-windows-latest-chromium
❌ [chromium] › mcp/dashboard.spec.ts:264 › should annotate when context has no fixed viewport @mcp-windows-latest-chromium
❌ [chromium] › mcp/dashboard.spec.ts:301 › should cancel browser_annotate when the MCP request is aborted @mcp-windows-latest-chromium
❌ [chromium] › mcp/dashboard.spec.ts:332 › should cancel browser_annotate when the MCP client disconnects @mcp-windows-latest-chromium
❌ [chromium] › mcp/dashboard.spec.ts:361 › should switch screencast to -s session on show --annotate @mcp-windows-latest-chromium
❌ [chromium] › mcp/dashboard.spec.ts:410 › should disengage annotate mode when --annotate client disconnects @mcp-windows-latest-chromium
❌ [chromium] › mcp/dashboard.spec.ts:477 › save recording streams WebM bytes to the chosen file @mcp-windows-latest-chromium
❌ [webkit] › mcp/dashboard.spec.ts:164 › should capture annotations via show --annotate @mcp-windows-latest-webkit
❌ [webkit] › mcp/dashboard.spec.ts:185 › should start dashboard and annotate when no dashboard is running @mcp-windows-latest-webkit
❌ [webkit] › mcp/http.spec.ts:103 › http transport browser lifecycle (isolated) @mcp-windows-latest-webkit

6892 passed, 1052 skipped

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Merge workflow run.

[github-actions]

Test results for "tests 1"

3 flaky

⚠️ [chromium-library] › library/video.spec.ts:719 › screencast › should work with video+trace `@chromium-ubuntu-22.04-node22`
⚠️ [firefox-library] › library/inspector/cli-codegen-3.spec.ts:224 › cli codegen › should generate frame locators (4) `@firefox-ubuntu-22.04-node20`
⚠️ [webkit-page] › page/page-set-input-files.spec.ts:38 › should upload a folder `@webkit-ubuntu-22.04-node20`

41632 passed, 850 skipped

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Merge workflow run.