Python: Phase 3: Core Vector Store Abstractions #4165

Parent: #1188
Depends on: #4163

Goal

Establish all vector store types, enums, the decorator, collection definition, and base classes.

Tasks

• Vector store enums and field types in _vectors.py (FieldTypes, IndexKind, DistanceFunction)
• VectorStoreField, VectorStoreCollectionDefinition, SearchOptions, SearchResponse, RecordFilterOptions
• @vectorstoremodel decorator
• Serialization/deserialization protocols
• BaseVectorCollection, BaseVectorStore, BaseVectorSearch base classes
• SupportsVectorUpsert, SupportsVectorSearch protocols
• create_search_tool on BaseVectorSearch
• Exception types
• Tests for all vector store abstractions

Mergeable: Yes — adds new abstractions, no breaking changes.