get_overrides_for_block and get_overrides_for_course return reset/reverted overrides

Description

When a due date extension is reset via set_date_for_block(course_id, block_id, 'due', None, user=student), a new UserDate record is created with abs_date=None and rel_date=None. The actual_date property on this record falls back to the original ContentDate policy date, making it indistinguishable from an active extension.

Both get_overrides_for_block and get_overrides_for_course return these reset records because they query all UserDate rows (most recent per user/block) without checking whether the override actually differs from the base date.

This forces every consumer to independently filter out reset overrides. Currently, edx-platform works around this in the v2 instructor API (UnitExtensionsView): https://github.com/openedx/openedx-platform/pull/38394

Expected behavior

get_overrides_for_block and get_overrides_for_course should only return overrides that represent actual date changes, not reverted/reset records.

Related issues

https://github.com/openedx/frontend-app-instructor-dashboard/issues/160
https://github.com/openedx/openedx-platform/issues/38393

Start by reading get_overrides_for_block and get_overrides_for_course, then trace how they select the most recent UserDate records and calculate actual_date. Verify behavior with existing tests if present; done means reset records with no date change are excluded while active overrides remain returned.