feat(expect): support expect.soft.poll chain

[pavelfeldman]

Summary

• expect.soft.poll(...) now works as a shorthand for expect.configure({ soft: true }).poll(...). expect.soft is now a fully chainable Expect instance, so .poll, .configure, .extend, etc. all work after .soft.

Fixes #20773

[github-actions]

Test results for "MCP"

7086 passed, 1104 skipped

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Merge workflow run.

[github-actions]

Test results for "tests 1"

3 flaky

⚠️ [chromium-library] › library/popup.spec.ts:261 › should not throw when click closes popup `@chromium-ubuntu-22.04-arm-node20`
⚠️ [chromium-library] › library/video.spec.ts:647 › screencast › should capture full viewport `@chromium-ubuntu-22.04-node24`
⚠️ [chromium-library] › library/popup.spec.ts:261 › should not throw when click closes popup `@chromium-ubuntu-22.04-node22`

41877 passed, 850 skipped

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Merge workflow run.