Converting TypeMoq tests to sinon

[Benjin]

First batch of TypeMoq -> Sinon test conversions, using AI with a good bit of coaching.

I've rolled most of the prompt/rules I've been using into test/unit/AGENTS.md.

Code Changes Checklist

• New or updated unit tests added
• All existing tests pass (npm run test)
• Code follows contributing guidelines
• Telemetry/logging updated if relevant
• No regressions or UX breakage

Reviewers: Please read our reviewer guidelines

[github-actions]

PR Changes

Category	Target Branch	PR Branch	Difference
Code Coverage	55.72%	55.79%	⚪ 0.00%
VSIX Size	4856 KB	4856 KB	⚪ 0 KB ( 0% )
Webview Bundle Size	4008 KB	4008 KB	⚪ 0 KB ( 0% )

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.91%. Comparing base (4f08005) to head (83c3042).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20274      +/-   ##
==========================================
+ Coverage   53.83%   53.91%   +0.07%     
==========================================
  Files         198      198              
  Lines       17309    17309              
  Branches     1153     1151       -2     
==========================================
+ Hits         9319     9332      +13     
+ Misses       7990     7977      -13     

see 4 files with indirect coverage changes

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