[19.0][MIG] hr_expense_tier_validation: Migration to 19.0

[dnplkndll]

Port of hr_expense_tier_validation to 19.0.

Non-mechanical adaptations worth flagging

• Line-Level Tier Validation: Refactored hr_expense_tier_validation to inherit from tier.validation and apply multi-step approval gates directly on individual hr.expense records instead of the removed hr.expense.sheet core model.
• Search Filters: Ported search filter criteria like "Needs my Review" and "Validated" to target hr.expense search views.
• Test Modernisation: Rewrote unit tests to verify tier validation rules and constraint checks directly on individual hr.expense records in the submitted state.

[dnplkndll]

Replaced by #349 — source branch moved from dnplkndll fork to ledoent organization fork. Same commits (HEAD b13d317), no content change.