[geaflow/ai-memory] Add graph backend conformance tests

Priority: P0
Difficulty: Intermediate

Context: Multiple backends must behave consistently for Graph Memory retrieval, deletion, and replay.

Scope:

- Add backend contract test suite.
- Cover duplicate vertex, parallel edge, self-loop, dangling edge rejection, delete, restart, scan order normalization.

Constraints:

- Tests must run against fake/local backend without external services.
- Do not encode implementation-specific ordering unless explicitly sorted.

Acceptance Criteria:

- Fake backend and local backend can both run the same test suite.
- Failures identify contract name and backend name.

Suggested paths:

- `geaflow-ai/src/test/java/org/apache/geaflow/ai/backend`

Start in geaflow-ai/src/test/java/org/apache/geaflow/ai/backend and inspect the existing fake and local backend test setup. Run the relevant tests to see how both backends are selected. Done means one shared suite covers duplicate vertices, parallel edges, self-loops, dangling-edge rejection, deletion, restart, and normalized scan order, with failures naming the contract and backend.

Written by the indexing model from the issue text.