feat(namespace): add search_many

[arshsk16]

Adds NamespaceView.search_many() to expose batched concurrent search while keeping queries pinned to the namespace.

Tests cover:

• multiple queries with namespace isolation
• forwarding top_k and filter kwargs
• empty query batches

All local test pass

Fixes #244

This change was prepared with AI assistance under human direction and review.

[Kaap10]

LGTM! Great work @arshsk16. The search_many implementation on NamespaceView is clean and test_namespace.py covers all edge cases nicely.