Add Network Inspector server unit tests

[ConnorQi01]

Summary

• Add direct Network Inspector server unit tests.
• Cover certificate exchange medium conversion, secure config load failures, server start/stop lifecycle, and async connection registration/removal.

Validation

• node .\node_modules\mocha\bin\_mocha test\extension\networkInspector\networkInspectorServer.test.js --ui tdd --timeout 10000
• npm run build
• npm test downloaded the VS Code test archive, then the local Windows test host failed to start with Error mutex already exists and EPIPE.

Closes #2694