test: add AssociativeMemoryBank coverage

[xiaokhkh]

Summary

Adds focused unit coverage for AssociativeMemoryBank without changing production behavior.

Covered behavior:

• duplicate suppression by default
• duplicate retention with allow_duplicates=True
• newline normalization during add
• associative retrieval ordering with deterministic embeddings
• recent retrieval window ordering
• non-positive retrieval limit validation
• get_state / set_state preserving pending memories and duplicate hashes

Part of #205.

Testing

• PYTHONPATH=. python -m pytest -q concordia/associative_memory/basic_associative_memory_test.py
• PYTHONPATH=. python -m pylint --errors-only concordia/associative_memory/basic_associative_memory_test.py
• python -m pyink --check concordia/associative_memory/basic_associative_memory_test.py
• PYTHONPATH=. python -m pytype concordia/associative_memory/basic_associative_memory.py concordia/associative_memory/basic_associative_memory_test.py

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[jzleibo]

Looks good to me