Add reduce_to_unit method to GaussValuation, fixes #39702

[5iri]

This pull request addresses issue #39702 by adding a reduce_to_unit method to the GaussValuation class in SageMath. The method simplifies reducing the unit part of an element to valuation 0, which was previously a tedious process.

Changes Made:

• Added reduce_to_unit method to src/sage/rings/valuation/gauss_valuation.py.
• The method shifts the element's valuation to 0 using equivalence_unit and then reduces it.
• Fixed handling of infinite valuations by comparing with infinity from sage.rings.infinity.