Align backend (BE) and frontend (FE) validation for staff only filings

This epic aims to align backend and frontend validation logic for enabled features to ensure a consistent and predictable user experience during API user onboarding. By unifying validation rules, we reduce the risk of mismatches between the UI and API behavior, simplify troubleshooting, and provide clearer feedback to users. This alignment is a critical step toward streamlining and scaling the onboarding process for the API users (Group 2 onboarding).

For anything involving data validation and business logic, the solution should primarily be backend/API-driven, with the front-end acting as a supportive layer (e.g. for UX).

Priority : Validation logic for the features that are enabled in Prod.

Notes:
This work started as part of Sandbox development - see tickets in https://app.zenhub.com/workspaces/entities---olga-65af15f59e89f5043c2911f7/issues/gh/bcgov/entity/27848

See Detailed analysis worksheet from Sandbox:
https://docs.google.com/spreadsheets/d/1N5zs291nvPpHbGCxJS9jmxX3yoUyG_mi-906j-_TuHo/edit?gid=377655930#gid=377655930

New spreadsheet to track Analysis by Filing:
https://docs.google.com/spreadsheets/d/1qhe_lkBQyiiNKBBdPxM5v5Ub6lGuBK1QrC33yJRnV3U/edit?usp=sharing

Tasks:

- [x] Assess what's already aligned between Frontend and Backend from the sandbox phase.
- [ ] Identify gaps or mismatches between FE, BE, and the API for features enabled for Group 2 onboarding
- [ ] Address the difference
- [ ] Add necessary logging for BE validation (coordinate this work with https://app.zenhub.com/workspaces/entities---olga-65af15f59e89f5043c2911f7/issues/gh/bcgov/entity/29794)

**Filings:**
**- All staff only filings**

Related Epics:
Initial Filings: https://app.zenhub.com/workspaces/entities-team-65af15f59e89f5043c2911f7/issues/gh/bcgov/entity/29804
Other Public Filings: https://app.zenhub.com/workspaces/entities-team-65af15f59e89f5043c2911f7/issues/gh/bcgov/entity/32548

Start with the completed Sandbox assessment and the Detailed analysis worksheet, then compare the frontend, backend, and API validation for enabled staff-only filings. Track mismatches in the new Analysis by Filing spreadsheet and coordinate backend validation logging with issue 29794. Done means the identified gaps are addressed and the validation behavior is aligned.

Written by the indexing model from the issue text.