Add docstring to similarity_search_by_vector method

[Copilot]

Issue Key

Summary

Adds comprehensive docstring to similarity_search_by_vector method following the established pattern from similarity_search_with_score.

Changes in this PR

• Added docstring to MongoDBAtlasVectorSearch.similarity_search_by_vector() documenting:
  • Method purpose and Atlas Vector Search context
  • Parameters: query_vector, k, pre_filter, post_filter_pipeline, oversampling_factor, include_embeddings, kwargs
  • Return value: List of Document objects

Test Plan

No functional changes to code. Docstring format validated with ruff linter to match codebase style conventions.

Screenshots (optional)

N/A

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is the intention of the code captured in relevant tests?
• If there are new TODOs, has a related JIRA ticket been created?
• Has a MongoDB Employee run the patch build of this PR?

Checklist for Reviewer {@primary_reviewer}

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Have you checked for spelling & grammar errors?
• Is all relevant documentation (README or docstring) updated?

Focus Areas for Reviewer (optional)

N/A - documentation-only change

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