Add unit tests for RunEasBuild command - #2853

Summary

• add focused proxyquire/Sinon coverage for RunEasBuild file creation paths
• verify existing files are preserved and permission failures show the expected error

Validation

• npx prettier --check test\extension\commands\runEasBuild.test.ts
• npm run build
• npx mocha --ui tdd test\extension\commands\runEasBuild.test.js (6 passing)
• npm test (205 tests executed; 4 pre-existing environment/Extension Host failures outside RunEasBuild)

Closes #2840