[geaflow/ai-memory] Implement schema-first property graph extraction prompt builder

Priority: P1
Difficulty: Intermediate

Context: Later LLM extraction needs consistent prompts, but prompt construction should be isolated from model clients. This issue provides prompt generation, not model execution.

Scope:

- Add `ExtractionPromptBuilder`.
- Generate prompt sections for schema, allowed entity types, allowed relation types, examples, and output JSON format.
- Add golden prompt tests.

Constraints:

- No remote model call.
- Prompt must include strict JSON output requirement and source span requirement.
- Prompt must not include unrelated schemas.

Acceptance Criteria:

- Prompt output is deterministic for the same schema and examples.
- Golden tests cover English, Chinese, and mixed text.
- Prompt builder rejects schema with no vertex or edge definitions.

Suggested paths:

- `geaflow-ai/src/main/java/org/apache/geaflow/ai/extract/prompt`

Start in geaflow-ai/src/main/java/org/apache/geaflow/ai/extract/prompt and identify the prompt-builder entry point and nearby schema types. Add golden prompt tests covering English, Chinese, and mixed text, then verify deterministic output, strict JSON and source-span requirements, rejection of schemas without vertex or edge definitions, and exclusion of unrelated schemas.

Written by the indexing model from the issue text.