Update: replace removed Thread.stop() with stream-close cancellation

[gian21391]

Download cancellation in Update relied on Thread.stop(), which is removed in
recent JDKs and already throws UnsupportedOperationException at runtime on the
JDK 21 target, so cancelling was a silent no-op. The download watcher now closes
the source stream on cancel to abort the in-progress Files.copy. This is a step
towards improving support for JDK 26, where the old code no longer compiles.

Checklist

• Testing:

  • The feature has automated tests
  • Tests were run
  • If not, explain how you tested your changes

• Documentation:

  • The feature is documented
  • The documentation is up to date
  • Release notes:
    • Added an entry if the change is breaking or significant
    • Added an entry when adding a new feature

[sonarqubecloud]

Quality Gate passed

Issues
4 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[georgweiss]

@kasemir, any reason to not merge?

[kasemir]

No, go ahead