[AVM CI Environment Issue]: Pester test: types MUST have `@description` decorator

### Check for previous/existing GitHub issues

- [x] I have checked for previous/existing GitHub issues

### Issue Type?

Feature Request

### Description

Add a static validation Pester test to ensure @description decorator on UDTs, including:
- main type (UDT) definition
- All type properties
- discriminator property

Ref spec [BCPNFR21](https://azure.github.io/Azure-Verified-Modules/spec/BCPNFR21/)
Ref thread: https://github.com/Azure/bicep-registry-modules/pull/4146/files#r1995837313

This should be implemented in 2 steps

1. Soft failure, allowing discovery of the missing spec across the library and relative existing module fixes
2. Enforcement, ensuring the spec compliance for all new modules

Start with the repository's Pester test suite and read spec BCPNFR21 to confirm the required @description decorators for UDTs, their properties, and discriminator properties. Implement the requested soft-failure discovery step first, then consider enforcement for new modules; done means the test identifies existing violations and ultimately blocks non-compliant additions.

Written by the indexing model from the issue text.