feat: add QueryExtensions for filtering by Envelope and Receiver

- 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.

EnvelopeGenerator.Domain/Interfaces/IHasEnvelope.cs

@@ -5,7 +5,7 @@
     {
 #endif
 
-interface IHasEnvelope
+public interface IHasEnvelope
 {
 #if NET
     public 

EnvelopeGenerator.Domain/Interfaces/IHasReceiver.cs

@@ -5,7 +5,7 @@
     {
 #endif
 
-interface IHasReceiver
+public interface IHasReceiver
     {
 #if NET
     public