[dexter] Correctly identify stop-reason while driving VisualStudio (#94754)

Prior to this patch VisualStudio._get_step_info incorrectly identifies
the reason the debugger has stopped. e.g., stepping through a program
would be reported as a StopReason.Breakpoint rather than
StopReason.Step.

Fix. No test added as there are no VisualStudio tests (tested locally).