GH-95704: Don't suppress errors from tasks when TG is cancelled (#95761)

When a task catches CancelledError and raises some other error,
the other error should not silently be suppressed.

Any scenario where a task crashes in cleanup upon cancellation
will now result in an ExceptionGroup wrapping the crash(es)
instead of propagating CancelledError and ignoring the side errors.

NOTE: This represents a change in behavior (hence the need to
change several tests).  But it is only an edge case.

Co-authored-by: Thomas Grainger <tagrain@gmail.com>