fix(tracing): mkdir tracesDir before live .stacks writes

[Skn0tt]

Fixes a race in live tracing.

localUtils.tracingStarted returns a stacksId at <tracesDir>/<traceName>.stacks, and addStackToTracingNoReply writes to that file directly. The recorder’s Tracing enqueues a recursive mkdir of <tracesDir>/resources through its own SerializedFS queue — separate from the per-session writer chain in localUtils, so there’s no happens-before guarantee between them.

When the SerializedFS worker hasn’t drained the mkdir yet and a stack write fires (live mode + an action like page.goto), the write fails with ENOENT.

Surfaces as a flake on mcp-windows-latest-firefox via tests/mcp/tracing.spec.ts:54 (browser_stop_tracing returns result: undefined because tracing.stop bubbled the ENOENT).

Fix: precreate path.dirname(traceStacksFile) in tracingStarted, so the contract is self-contained for any caller.

https://mspwblobreport.z1.web.core.windows.net/run-25561393525-1-d8c56b0253640b9f180ee09093c38892d31661d7/index.html#?testId=39a796cef0e6f5869c06-d52e545eb08e8dd5c09c7

[github-actions]

Test results for "MCP"

9 failed
❌ [firefox] › mcp/annotate.spec.ts:173 › user-initiated annotate downloads zip with feedback.md @mcp-ubuntu-latest-firefox
❌ [firefox] › mcp/cli-devtools.spec.ts:217 › video-start-stop @mcp-windows-latest-firefox
❌ [firefox] › mcp/cli-devtools.spec.ts:231 › video-chapter @mcp-windows-latest-firefox
❌ [msedge] › mcp/annotate.spec.ts:137 › should abort MCP annotation when last screenshot is removed @mcp-windows-latest-msedge
❌ [msedge] › mcp/annotate.spec.ts:273 › should enter annotate mode on fresh dashboard.tsx mount with -s --annotate @mcp-windows-latest-msedge
❌ [msedge] › mcp/annotate.spec.ts:330 › should annotate when context has no fixed viewport @mcp-windows-latest-msedge
❌ [msedge] › mcp/annotate.spec.ts:367 › should cancel browser_annotate when the MCP request is aborted @mcp-windows-latest-msedge
❌ [msedge] › mcp/annotate.spec.ts:398 › should cancel browser_annotate when the MCP client disconnects @mcp-windows-latest-msedge
❌ [webkit] › mcp/launch.spec.ts:101 › isolated context @mcp-windows-latest-webkit

7022 passed, 1068 skipped

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Merge workflow run.

[github-actions]

Test results for "tests 1"

4 flaky

⚠️ [chromium-library] › library/video.spec.ts:647 › screencast › should capture full viewport `@ubuntu-22.04-chromium-tip-of-tree`
⚠️ [chromium-library] › library/video.spec.ts:682 › screencast › should capture full viewport on hidpi `@ubuntu-22.04-chromium-tip-of-tree`
⚠️ [chromium-library] › library/video.spec.ts:719 › screencast › should work with video+trace `@chromium-ubuntu-22.04-arm-node20`
⚠️ [firefox-library] › library/inspector/cli-codegen-3.spec.ts:224 › cli codegen › should generate frame locators (4) `@firefox-ubuntu-22.04-node20`

41692 passed, 850 skipped

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Merge workflow run.