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.Application/Extensions/QueryExtensions.cs

@@ -1,8 +1,63 @@
-namespace EnvelopeGenerator.Application.Extensions;
+using DigitalData.Core.Exceptions;
+using EnvelopeGenerator.Application.Model;
+using EnvelopeGenerator.Domain.Interfaces;
+
+namespace EnvelopeGenerator.Application.Extensions;
 
 /// <summary>
 /// 
 /// </summary>
 public static class QueryExtensions
 {
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <typeparam name="TEntity"></typeparam>
+    /// <typeparam name="TEnvelopeQuery"></typeparam>
+    /// <param name="root"></param>
+    /// <param name="query"></param>
+    /// <param name="notnull"></param>
+    /// <returns></returns>
+    /// <exception cref="BadRequestException"></exception>
+    public static IQueryable<TEntity> Where<TEntity, TEnvelopeQuery>(this IQueryable<TEntity> root, IHasEnvelopeQuery<TEnvelopeQuery> query, bool notnull = true)
+        where TEntity : IHasEnvelope where TEnvelopeQuery : EnvelopeQueryBase
+    {
+        if (query.Envelope.Id is not null)
+            root = root.Where(e => e.Envelope!.Id == query.Envelope.Id);
+        else if (query.Envelope.Uuid is not null)
+            root = root.Where(e => e.Envelope!.Uuid == query.Envelope.Uuid);
+        else if (notnull)
+            throw new BadRequestException(
+                "Either Envelope Id or Envelope Uuid must be provided in the query."
+            );
+
+        return root;
+    }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <typeparam name="TEntity"></typeparam>
+    /// <typeparam name="TReceiverQueryBase"></typeparam>
+    /// <param name="root"></param>
+    /// <param name="query"></param>
+    /// <param name="notnull"></param>
+    /// <returns></returns>
+    /// <exception cref="BadRequestException"></exception>
+    public static IQueryable<TEntity> Where<TEntity, TReceiverQueryBase>(this IQueryable<TEntity> root, IHasReceiverQuery<TReceiverQueryBase> query, bool notnull = true)
+        where TEntity : IHasReceiver where TReceiverQueryBase : ReceiverQueryBase
+    {
+        if (query.Receiver.Id is not null)
+            root = root.Where(e => e.Receiver!.Id == query.Receiver.Id);
+        else if (query.Receiver.EmailAddress is not null)
+            root = root.Where(e => e.Receiver!.EmailAddress == query.Receiver.EmailAddress);
+        else if (query.Receiver.Signature is not null)
+            root = root.Where(e => e.Receiver!.Signature == query.Receiver.Signature);
+        else if (notnull)
+            throw new BadRequestException(
+                "Receiver must have at least one identifier (Id, EmailAddress, or Signature)."
+            );
+
+        return root;
+    }
 }

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