[geaflow/ai-memory] Define `ExtractionSchema` for property graph and KBQA triples

Priority: P0
Difficulty: Intermediate

Context: HugeGraph-AI-style import uses schema/prompt-driven extraction. geaflow-ai currently supports pre-structured graph import, but lacks a workflow that converts text paragraphs into typed graph facts or traditional KBQA triples.

Scope:

- Add `ExtractionSchema` contract for vertex types, edge types, properties, aliases, required fields, and allowed relation predicates.
- Include a KBQA triple projection: `subject`, `predicate`, `object`, `source_span`, `confidence`.
- Add JSON examples and parser tests.

Constraints:

- Do not implement LLM extraction in this issue.
- Do not hard-code a single business ontology.
- Schema parsing must fail closed for unknown required fields.

Acceptance Criteria:

- Valid schema examples round-trip through JSON.
- Invalid relation predicate, missing required property, and duplicate type definitions fail with typed errors.
- Schema examples include both property graph and triple-style facts.

Suggested paths:

- `geaflow-ai/src/main/java/org/apache/geaflow/ai/extract/schema`
- `geaflow-ai/src/test/resources/extraction/schema`

Start in geaflow-ai/src/main/java/org/apache/geaflow/ai/extract/schema and review the schema contract requirements, then inspect geaflow-ai/src/test/resources/extraction/schema for the expected JSON examples. Add parser tests covering round trips and typed failures for invalid predicates, missing required properties, duplicate definitions, and unknown required fields; done means both property-graph and triple-style examples validate correctly.

Written by the indexing model from the issue text.