[geaflow/ai-memory] Add `EntityVectorIndex` wrapper around graph entity embeddings

[kitalkuyo-gita]

Priority: P0
Difficulty: Intermediate

Context: Graph entity embedding exists, but Phase 1 needs explicit entity-vector semantics for fuzzy entity anchoring.

Scope:

• Wrap current EmbeddingIndexStore behavior behind EntityVectorIndex.
• Add entity ID, entity type, model metadata, and vector version.
• Add compatibility tests with existing EmbeddingOperator.

Constraints:

• Do not change retrieval ranking yet.
• Link existing embedding correctness fix issue/PR where applicable.

Acceptance Criteria:

• Entity vector lookup can distinguish vertex and edge entities.
• Dimension/model mismatch is detected before search.

Suggested paths:

• geaflow-ai/src/main/java/org/apache/geaflow/ai/index/entity