fix: stop server and clean up ports on extension deactivate

[TPreece101]

Summary

• When the extension was uninstalled/reinstalled, the old HTTP server kept running, causing the new instance to fall back to a different port
• Root cause: deactivate() had no reference to the DebugServer instance (local variable inside activate())
• Added module-level refs, a disposable safety net in context.subscriptions, and a proper deactivate() implementation

Test plan

• npm run compile — no errors
• npm test — all 72 tests pass
• Manual: start extension, note port, uninstall, reinstall — server starts on the same port without a port conflict notification

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.70%. Comparing base (73f4b29) to head (7d726aa).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #19      +/-   ##
==========================================
+ Coverage   74.30%   75.70%   +1.39%     
==========================================
  Files           7        7              
  Lines        1195     1210      +15     
  Branches      151      153       +2     
==========================================
+ Hits          888      916      +28     
+ Misses        306      292      -14     
- Partials        1        2       +1     

Flag	Coverage Δ
extension	75.70% <100.00%> (+1.39%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.