test: add VSCode extension testing (#22)

[newtontech]

Summary

This PR adds comprehensive VSCode extension testing infrastructure as requested in issue #22.

Changes

Test Files Added

• src/test/suite/extension.test.ts - Comprehensive test suite with 21 test cases

Tests Implemented

1. Extension Activation Tests

   • Extension presence verification
   • Extension activation verification
   • Sidebar context setting verification

2. Command Registration Tests (21 commands total)

   • openqc.visualizeStructure
   • openqc.plotData
   • openqc.previewInput
   • openqc.startLSP
   • openqc.stopLSP
   • openqc.restartLSP
   • openqc.validate
   • openqc.sidebar.refreshMolecules
   • openqc.sidebar.refreshJobs
   • openqc.sidebar.openMolecule
   • openqc.sidebar.deleteMolecule
   • openqc.sidebar.runCalculation
   • openqc.sidebar.viewResults
   • openqc.sidebar.exportData
   • openqc.sidebar.cancelJob
   • openqc.sidebar.restartJob
   • openqc.convertFormat
   • openqc.convertToXYZ
   • openqc.convertToPDB
   • openqc.convertToVASP
   • openqc.convertToGaussian
   • openqc.batchConvert
   • openqc.checkConverterBackend

Dependencies

• Added mocha@^10.7.3 to devDependencies for test runner

Test Framework

• Uses existing @vscode/test-electron framework
• Follows VSCode extension testing best practices
• Tests run in TDD mode with Mocha

Testing

The test suite can be run with:

npm run test:e2e

Checklist

• Test files created in src/test/suite/
• Extension activation tests implemented
• Command registration tests implemented
• Dependencies added to package.json
• Code follows project conventions
• Commit message follows conventional commits

Fixes #22