Move externalEngines.delete(projectId) to immediately after acquiring the
singleton lock instead of after engine.start() succeeds. If a project was
marked external, the holder exits, acquire succeeds, but start() then throws,
the success-path delete never ran and hasRunningEngine() reported a phantom
engine forever. Added a regression test for the failed-takeover path.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
fa9a3cc52f