Refactor BitVector array type "Q" to a constant ARRAY_TYPE

[google-labs-jules]

This PR refactors the hardcoded "Q" array type code used throughout BitVector.py into a single module-level constant named ARRAY_TYPE. This improves maintainability and centralizes the configuration of the underlying array storage format. All occurrences of array.array("Q", ...) have been updated to array.array(ARRAY_TYPE, ...). Tests have been run to ensure no regressions were introduced.

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PR created automatically by Jules for task 11543415901475398441 started by @schwehr

[google-labs-jules]

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