[Feature] Add unit tests for RunEasBuild command

[ConnorQi01]

Summary

The RunEasBuild command (src/extension/commands/runEasBuild.ts) has no unit test coverage. It contains conditional file-creation logic that is worth validating: it creates eas.json when absent, creates .eas/workflows/create-production-builds.yml when absent, and shows error messages on file-system permission failures. None of this logic is currently exercised by any test.

Why this is useful

Unit tests will catch regressions in the file-creation and permission-error paths without requiring a real EAS project, keeping CI reliable as the command evolves alongside the EAS workflow feature.

Suggested scope

• Mock the file system (fs.existsSync, fs.writeFileSync, fs.mkdirSync) and vscode.window.showErrorMessage using proxyquire + Sinon, following the same pattern as launchAndroidEmulator.test.ts
• Test case: project root path is undefined → error message shown, no file created
• Test case: eas.json already exists → file write skipped
• Test case: eas.json missing → file created with {}
• Test case: workflow file already exists → mkdirSync/writeFileSync skipped
• Test case: workflow file missing → directory and file created with expected YAML template
• Test case: fs.writeFileSync throws (permission error) → error message shown

Evidence

• Source file: src/extension/commands/runEasBuild.ts
• No corresponding test file exists in test/extension/commands/
• Recent commits show an established pattern: 0446243a, ea74c926, 1af55735 each add a focused test file for a single command
• Open follow-up issues for similar work: [Feature] Add unit tests for LaunchIOSSimulator command #2834, [Feature] Add unit tests for LaunchExpoWeb command #2827, [Feature] Add unit tests for OpenExpoUpgradeHelper command #2838

Validation

• Run npm test (or the mocha suite for test/extension/commands/) and confirm the new test file passes
• Confirm no regressions in the existing command test suite