Rename schema to collection to match Weaviate's updated terminology

[Copilot]

Weaviate now refers to schemas as "collections." This aligns the Ruby client's public API with that naming while preserving full backward compatibility.

Changes

• New Weaviate::Collection class (lib/weaviate/collection.rb) — primary class replacing Schema
• Weaviate::Schema aliased to Collection — existing code continues to work without changes
• client.collections — new primary accessor on Client; client.schema delegates to it for backward compat
• spec/weaviate/collection_spec.rb — full test coverage for Collection
• README — updated all examples to use collections

Usage

# New (preferred)
client.collections.create(class_name: 'Question', ...)
client.collections.list
client.collections.get(class_name: 'Question')
client.collections.delete(class_name: 'Question')

# Old (still works)
client.schema.create(class_name: 'Question', ...)