Add PrebuildClean command unit test

[ConnorQi01]

Summary

Add unit coverage for the Expo Prebuild Clean command.

Proposed Changes

• Add a PrebuildClean command unit test mirroring the existing Prebuild command coverage.
• Verify the command uses the Expo Prebuild Clean logger channel.
• Verify the node modules root lookup and command executor invocation use the project root.
• Verify the exact command string is
  px expo prebuild --clean.

Test Plan

pm run build

pm test (fails in existing
n-extension commandsRegistered test because the registered command list includes
eactNative.publishToExpHost; the new prebuildCleanCommand test passes.)

Closes #2755