[Feature] Add unit tests for PrebuildClean command

[ConnorQi01]

Summary

PrebuildClean (src/extension/commands/prebuildClean.ts) has no unit tests, while its sibling command Prebuild already has test coverage in test/extension/commands/prebuild.test.ts. The two commands share the same structure — both use CommandExecutor, AppLauncher.getNodeModulesRootByProjectPath, and OutputChannelLogger.getChannel — but PrebuildClean is missing even a basic execution path test.

Why this is useful

Keeps Expo command test coverage consistent: every CommandExecutor-based command should verify that it calls the correct CLI command from the correct project root. A future refactor of prebuildClean.ts would silently regress without a test.

Suggested scope

• Add test/extension/commands/prebuildClean.test.ts mirroring the structure of prebuild.test.ts
• Verify that commandExecutor.execute is called with "npx expo prebuild --clean"
• Verify the logger channel is opened with the "Expo Prebuild Clean" label
• Verify that AppLauncher.getNodeModulesRootByProjectPath is called with the correct project root

Evidence

• src/extension/commands/prebuildClean.ts — no corresponding test file exists
• test/extension/commands/prebuild.test.ts — identical structure, already covered
• git log --oneline -30 — steady pattern of adding sibling tests for each command (enableHermes → enableExpoHermes, prebuild → ?, cleanRestartPackager, etc.)

Validation

• npm test passes with the new test file included
• Test stubs confirm npx expo prebuild --clean is the exact string passed to execute