Simplify retrieval by extracting vector search into a helper function

[gaurigupta100]

#Summary

• Extracted vector search logic from retrieve() into a separate _vector_search() helper.
• Kept the retrieval behavior unchanged.
• Improved readability and maintainability.

Fixes #189

Testing

pytest tests/test_retriever.py

Result:

12 passed

[github-actions]

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[Aharshi3614]

@gaurigupta100 Please resolve the merge conflicts