feat: resolve preferredname at datasource layer to eliminate firstname fallback checks

[William-Edwards-STFC]

Description

Previously, preferredname was nullable throughout the codebase, requiring a preferredname ? preferredname : firstname check everywhere a user's name was displayed. This change moves the fallback logic to the datasource layer so that preferredname is always populated, removing the need for these checks across the frontend and backend.

Motivation and Context

The repeated fallback checks were scattered across utility functions, components, and backend code we may as well just do it once. firstname is kept separate on the model so it remains available for use in official documents.

How Has This Been Tested

• Existing Cypress E2E tests in userAdministration.cy.ts cover the two key scenarios:
• A user with preferredname set — verifies the preferred name is shown in proposals
• A user with no preferredname — verifies the first name is shown as fallback
• New unit tests added to StfcUserDataSource.spec.ts covering the STFC-specific fallback from firstNameKnownAs to givenName, including the empty string case

Fixes

UserOfficeProject/issue-tracker#1499

Changes

• New feature — preferredname is now always resolved at the datasource layer
• Backend — preferredname is now a non-nullable field across models, GraphQL types, and resolvers
• Frontend — removed all preferredname ? preferredname : firstname fallback checks as they are no longer needed

Tests included/Docs Updated?

• I have added tests to cover my changes.
• All relevant doc has been updated

Issue number is 1499 got a bit mixed up whilst doing commits

[yoganandaness]

@William-Edwards-STFC I am a bit non inclined with this. The Datasource layer needs to be as much as less opinionated towards the business layer. It should always bring out the truth in the raw form from the database, without altering the data.

It is the business layer that should decide.

If you require a need for a preferred name to always have a non nullable value, then we need to discuss and fix it from ground zero(ie., change the column type from nullable to non-nullable and proceed with other layers)