Large-scale cleanup of entity and interface classes:
- Removed obsolete #if NET/NETFRAMEWORK preprocessor blocks
- Unified class/property formatting and indentation
- Standardized use of data annotations
- Improved nullable reference type handling
- Simplified interfaces (IHasEnvelope, IHasReceiver)
- Removed redundant and dead code
- No changes to business logic or data model
These changes modernize and standardize the codebase for easier maintenance and .NET compatibility.
- Introduced extension methods on IQueryable<TEntity> to filter entities
by Envelope (Id or Uuid) and Receiver (Id, EmailAddress, or Signature).
- Throws BadRequestException if no valid identifier is provided when notnull = true.
- Improves query handling consistency across application layer.
