[Unit Test Suite] Implement async unit tests for embedding

[aalbersk]

After implementing async support for the Embedding Microservice, some of the unit tests stopped to work because they started to require pytest-async e.g. these ones.

It would be beneficial for maintainability of the code to fix these tests or implement new ones to increase test coverage of the embedding microservice.

[pal0064]

E_RAG_Team3 will work on this