[geaflow/ai-memory] Harden keyword index serialization golden tests

[kitalkuyo-gita]

Priority: P0
Difficulty: Starter

Context: Keyword retrieval exists through Lucene/keyword vectors, but the main branch behavior has been described as rebuilding during query and vulnerable to noisy common tokens.

Scope:

• Add golden tests for keyword tokenization, serialization, reload, Unicode, stopwords, and common-token pollution.
• Ensure incremental add/delete expected behavior is documented.

Constraints:

• Do not implement a new keyword engine.
• Coordinate with any existing resident keyword index PR.

Acceptance Criteria:

• Golden tests fail if tokenization or serialization changes unexpectedly.
• Common tokens do not dominate exact graph anchoring fixtures.

Suggested paths:

• geaflow-ai/src/test/java/org/apache/geaflow/ai/index
• geaflow-ai/src/test/resources/index/keyword