DateTimeInput - code cleanup & alignment to grommet standards

We should go through the DateTimeInput code and make any cleanups and ensure it is consistent with our standards throughout the rest of the codebase.

Some items to look at:

• Adopt shared utility functions from sectionHelpers.js (unifying section-key and token lookups) instead of maintaining local reimplementations (DateTimeInput.js)

• Consolidate duplicated core utilities like normalizeStep and zero-padding helpers (pad2) into shared utility modules

• Align section limit definitions and digit handling with useSectionedTimeField to eliminate overlapping logic between date and time fields

• Refactor and simplify the architecture to fully leverage the segmented input model across both date and time sections (DateTimeInput.js)

• Add comprehensive regression tests covering segment navigation, keyboard parity, and accessibility compliance (DateTimeInput-test.tsx)

Review DateTimeInpue code and determine if additional cleanups are needed.

Anticipated effort: medium