[Geaflow/ai-memory] Define `DocumentSource` and `DocumentRecord` contracts

Priority: P0
Difficulty: Starter
Suggested labels: `area:ai-memory`, `type:contract`, `difficulty:starter`

Context: `TextFileReader` currently reads non-empty lines into strings. Graph Memory Phase 1 needs a stable document boundary before chunking, extraction, indexing, and replay can be deterministic.

Scope:

- Add contract classes for `DocumentSource`, `DocumentRecord`, and `SourceRef`.
- Include fields such as `source_id`, `uri`, `media_type`, `charset`, `content_hash`, `created_at`, `ingested_at`, and `metadata`.
- Add tests for empty files, duplicate source IDs, Unicode content, and stable content hash.

Constraints:

- Do not replace all existing readers in this issue.
- Do not add remote storage support.
- Do not store raw private text in logs.

Acceptance Criteria:

- A document loaded twice with the same content receives the same `content_hash`.
- Invalid or missing source metadata fails with a typed validation error.
- Existing `TextFileReader` tests remain compatible.

Suggested paths:

- `geaflow-ai/src/main/java/org/apache/geaflow/ai/graph/io`
- `geaflow-ai/src/test/java/org/apache/geaflow/ai`

Start in geaflow-ai/src/main/java/org/apache/geaflow/ai/graph/io and inspect the existing TextFileReader and its tests under geaflow-ai/src/test/java/org/apache/geaflow/ai. Define the DocumentSource, DocumentRecord, and SourceRef contracts with validation and stable hashing, then add coverage for empty files, duplicate source IDs, Unicode content, and repeated loads. Done means the acceptance criteria pass and existing TextFileReader tests remain compatible.

Written by the indexing model from the issue text.