fix(vector-stores): filter S3 vector list results

[haosenwang1018]

What changed

• Apply metadata filters client-side for S3 Vectors list() results.
• Keep pagination intact, then apply top_k after filtering so matching rows are not pre-truncated.
• Add a regression test for metadata filtering.

S3 Vectors list_vectors does not support server-side metadata filtering, so this preserves the existing API contract without changing the AWS request shape.

Fixes #3887.

Testing

• uv run --with pytest --with pytest-mock --with ruff --with boto3 python -m pytest tests/vector_stores/test_s3_vectors.py -q
• uv run --with ruff ruff check mem0/vector_stores/s3_vectors.py tests/vector_stores/test_s3_vectors.py