fix(aws): honor zero MemoryDB distance threshold

[WOLIKIMCHENG]

Summary

Treat distance_threshold=0.0 as an explicitly configured threshold when building MemoryDB vector queries.

• Use VECTOR_RANGE for every non-None threshold and bind the exact value.
• Use KNN only when the threshold is None.
• Preserve positive-threshold behavior.
• Add unit coverage for zero, positive, and absent thresholds.

Testing

• Focused unit tests: 3 passed
• Ruff check and format check
• Mypy on the changed source and test files
• Syntax, import, and diff hygiene checks