[geaflow/ai-memory] Add graph backend conformance tests #850

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