fix #4251: wrap BackgroundTasks in their own span (#4368)

* fix #4251: wrap BackgroundTasks in their own span

Only the request/ASGI middleware was traced; background tasks
were not.

Background tasks ran after the response with no span, causing
child spans to attach to a closed parent span.

This change patches BackgroundTask.__call__ to wrap execution
in a new span.

Each background task now creates a child span of the request,
ensuring a correct trace hierarchy.

Add three regression tests for background task instrumentation.

* Entry added to CHANGELOG.md linking to PR #4368

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Co-authored-by: Tammy Baylis <96076570+tammy-baylis-swi@users.noreply.github.com>
Co-authored-by: Aaron Abbott <aaronabbott@google.com>