ge-hch.5.15.17: add embedding service tests

[SorraTheOrc]

Summary

• add deterministic unit tests for embedding similarity thresholds (happy/joyful > 0.7, happy/database < 0.4)
• add embed(null) graceful fallback test and mocked worker plumbing test for embed()
• add optional gated integration test (INTEGRATION_EMBEDDING=1) to run the real model manually

Testing

• npx jest tests/unit/embedding.service.similarity.test.js tests/unit/embedding.service.embed.test.js --runInBand
• npx jest tests/unit/director.test.js --runInBand

Closes bd#ge-hch.5.15.17