[geaflow/ai-memory] Implement `SchemaValidator` for extracted graph facts

Priority: P0
Difficulty: Intermediate

Context: `GraphEntity` can be written today, but extracted facts need schema validation before entering graph storage or indexes.

Scope:

- Validate vertex labels, edge labels, required properties, property types, and relation endpoints.
- Return structured validation errors with fact ID and source span.
- Add tests for invalid label, missing property, wrong type, dangling edge, and unsupported predicate.

Constraints:

- Do not mutate the graph.
- Do not silently drop invalid facts.

Acceptance Criteria:

- Valid facts pass unchanged.
- Invalid facts are returned with stable error codes.
- Error messages do not include full raw document text.

Suggested paths:

- `geaflow-ai/src/main/java/org/apache/geaflow/ai/validate`

Start by tracing how GraphEntity enters graph storage or indexes, then inspect the suggested geaflow-ai/src/main/java/org/apache/geaflow/ai/validate package. Add focused tests for invalid labels, missing properties, wrong types, dangling edges, and unsupported predicates; done means valid facts pass unchanged and invalid facts return stable error codes without mutating the graph or exposing raw documents.

Written by the indexing model from the issue text.