Improve openTelemetry context propagation and user-land tracing in Vitest browser mode

Clear and concise description of the problem

Context

• https://github.com/vitest-dev/vitest/pull/9180#discussion_r2625566121

In browser mode, Vitest cannot automatically propagate openTelemetry async context due to lack of async_hooks. User-defined spans often lose parent context across async operations (promises, timers, events), leading to incomplete or flat tracing. Internal spans are manually linked, but user/test code loses context in deeply async code paths.

Suggested solution

Explore robust async context tracking for browsers (e.g. zone.js or context polyfills), expose helpers in Vitest to allow manual context handling for user code, and document current limitations and any recommended workarounds for reliable tracing.

Alternative

No response

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

Start with the discussion in PR #9180 and trace how Vitest browser mode handles user-defined spans across promises, timers, and events. Compare browser context-tracking options such as zone.js or context polyfills, then determine whether helpers or documentation are needed. Done means user-land tracing has a documented, reliable context-handling path or clearly stated limitations and workarounds.