fix: use default threading in Fastembed (#846)

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## Description
<!-- Provide a clear description of the changes in this PR -->
- set the parallel option to None in Fastembed's embedding function

## DCO Affirmation
I affirm that all code in every commit of this pull request conforms to
the terms of the Topoteretes Developer Certificate of Origin.

cognee/infrastructure/databases/vector/embeddings/FastembedEmbeddingEngine.py

@@ -44,7 +44,7 @@ class FastembedEmbeddingEngine(EmbeddingEngine):
                 embeddings = self.embedding_model.embed(
                     text,
                     batch_size=len(text),
-                    parallel=0,
+                    parallel=None,
                 )
 
                 return list(embeddings)