Preserve progress stop time

[mapitman]

Summary

• prevent progress tasks from overwriting the initial stop time
• add regression coverage for stop-time preservation

[Copilot]

Pull request overview

This PR prevents progress tasks from overwriting their initial stop time when StopTask() is called multiple times. The fix adds an early return guard in the StopTask() method that checks if the task is already stopped, preserving the original stop time and elapsed duration.

Changes:

• Added idempotency guard to StopTask() method to prevent overwriting stop time on subsequent calls
• Added regression test to verify stop time preservation when StopTask() is called multiple times

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/RipSharp/Utilities/SpectreProgressDisplay.cs	Added early return in StopTask() if task is already stopped to preserve the initial stop time
src/RipSharp.Tests/Utilities/SpectreProgressDisplayTests.cs	Added test case StopTask_DoesNotOverwriteStopTime to verify stop time is preserved when StopTask() is called multiple times