fix: use dash separator for S3 Vectors entity collection name

[PHclaw]

Summary

S3 Vectors does not support underscores in index/collection names. When entity linking is triggered during Memory.add(), the hardcoded _{collection_name}_entities separator causes a ValidationException from the S3 API.

Changes

• Added provider-aware separator: uses - (dash) for s3_vectors provider, keeps _ (underscore) for all other providers
• Applied to both entity_store property locations in main.py

Testing

Since I don't have S3 Vectors infrastructure, this was verified by code inspection. The fix is minimal and follows the suggestion in the issue.

Fixes #5361

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