apollo_propeller: rename DuplicateShard/ReceivedSelfPublishedShard variants

[sirandreww-starkware]

Note

Low Risk
Low risk: this is a pure rename of UnitValidationError variants (and associated call sites/tests) with no behavior changes, but it may break downstream pattern matches if any external crates rely on the old names.

Overview
Renames UnitValidationError variants to use unit terminology (ReceivedSelfPublishedShard -> ReceivedSelfPublishedUnit, DuplicateShard -> DuplicateUnit) and updates all internal references in PropellerScheduleManager::validate_origin, UnitValidator::validate_unit, and the unit validator tests to match.

^{Reviewed by Cursor Bugbot for commit 37d2616. Bugbot is set up for automated code reviews on this repo. Configure here.}

[reviewable-StarkWare]

This change is 

[guy-starkware]

@guy-starkware reviewed 4 files and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on noamsp-starkware and ShahakShama).

[cursor]

PR Summary

Low Risk
Purely a rename of error variants and call sites with no behavioral changes; risk is limited to missing references in downstream code.

Overview
Renames UnitValidationError variants to use unit terminology: ReceivedSelfPublishedShard -> ReceivedSelfPublishedUnit and DuplicateShard -> DuplicateUnit.

Updates origin validation (PropellerScheduleManager::validate_origin), unit validation (UnitValidator::validate_unit), and the corresponding validator test assertion to use the new error names.

^{Reviewed by Cursor Bugbot for commit 2ecd0b8. Bugbot is set up for automated code reviews on this repo. Configure here.}