Fix size calculation for FAISS Indexes

[CascadingRadium]

• Use the new FAISS API signature

[Copilot]

Pull request overview

Updates the Go FAISS bindings to use the newer FAISS C API signatures for retrieving index size, so Size() continues to work with updated FAISS versions.

Changes:

• Switch faiss_Index_size binding usage to the new (index, *size_t) -> int form and handle the return code.
• Switch faiss_IndexBinary_size binding usage to the new (index, *size_t) -> int form and handle the return code.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
index.go	Updates Index.Size() to call the new faiss_Index_size signature using an out-parameter.
index_binary.go	Updates BinaryIndex.Size() to call the new faiss_IndexBinary_size signature using an out-parameter.

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