feat(api): return Disposable from Tracing.start and Tracing.startChunk

[Pavel Feldman (pavelfeldman)]

Summary

• tracing.start() and tracing.startChunk() now return a Disposable, matching screencast.start(), tracing.startHar() and tracing.group().
• Disposing stops tracing (or the chunk) without saving, same as calling stop() / stopChunk() without a path.

[github-actions]

Test results for "tests 1"

4 flaky

⚠️ [chromium-library] › library/video.spec.ts:690 › screencast › should capture full viewport `@chromium-ubuntu-22.04-arm-node20`
⚠️ [chromium-page] › page/workers.spec.ts:191 › should attribute network activity for worker inside iframe to the iframe `@chromium-ubuntu-22.04-node24`
⚠️ [firefox-library] › library/browsercontext-cookies-third-party.spec.ts:257 › third party 'Partitioned;' cookies `@firefox-ubuntu-22.04-node20`
⚠️ [firefox-library] › library/browsercontext-cookies-third-party.spec.ts:470 › top level 'Partitioned;' cookie and same origin iframe `@firefox-ubuntu-22.04-node20`

52316 passed, 1243 skipped

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Merge workflow run.

[github-actions]

Test results for "MCP"

1 failed
❌ [firefox] › mcp/cli-core.spec.ts:97 › fill @mcp-windows-latest-firefox

8763 passed, 1480 skipped

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Merge workflow run.

[github-actions]

Hi, I'm the Playwright bot and I took a first look at the CI failures here.

🟢 The only failure is a known flake — the PR is clear

[firefox] › mcp/cli-core.spec.ts:97 › fill on mcp-windows-latest-firefox fails on main and on unrelated PRs too.

Details

This PR changes Tracing.start/Tracing.startChunk to return a Disposable: client/tracing.ts, types, docs, and tracing.spec.ts. None of that is on the MCP CLI fill path.

Pre-existing flake / infra

• [firefox] › mcp/cli-core.spec.ts:97 › fill (@mcp-windows-latest-firefox): a pre-existing flake. The test-results DB has 725 runs of this test. It ended red in 3 and a retry rescued it in 3 more. Every failure was on the Windows Firefox bot, and each time it failed with the same error: Expected: "- textbox [active] [ref=e2]: Hello, world!" / Received: undefined. Some of those failures:
  • main @ 2dec18d7e (run), d1ead3ecc (run), 8db9ed7bd (run)
  • PRs fix(mcp): disable the chromium sandbox for the bundled browser on linux #42490, test(webkit): reenable WebSocket handshake HAR coverage #42513, feat(evaluate): add "world" option to evaluate methods #42635

I couldn't refresh the DB for the latest runs because the GitHub API returned a 500. The failures above are already in the snapshot and are enough to confirm this is a flake.

_{Triaged by the Playwright bot - agent run}