Refactor command records and simplify document handling

- Updated `ReceiverGetOrCreateCommand` to include a required `IEnumerable<Signature>`.
- Modified `DocumentCreateCommand` to remove `DataAsByte` and enforce a required `DataAsBase64` property with immutability.
- Cleaned up SQL command formatting in `EnvelopeReceiverAddReadSQL`.
- Simplified document validation logic in `EnvelopeReceiverController` by removing redundant checks.
- Changed hardcoded connection string to a variable `_cnnStr` for improved security and flexibility.

EnvelopeGenerator.Application/EnvelopeReceivers/Commands/Create/CreateEnvelopeReceiverDtos.cs

@@ -36,14 +36,12 @@ public record ReceiverGetOrCreateCommand([Required] IEnumerable<Signature> Signa
 /// <summary>
 /// DTO zum Erstellen eines Dokuments.
 /// </summary>
-/// <param name="DataAsByte">
-/// Die Dokumentdaten im Byte-Array-Format. Wird verwendet, wenn das Dokument als Roh-Binärdaten bereitgestellt wird.
-/// </param>
-public record DocumentCreateCommand(byte[]? DataAsByte = null)
+public record DocumentCreateCommand()
 {
     /// <summary>
     /// Die Dokumentdaten im Base64-String-Format. Wird verwendet, wenn das Dokument als Base64-codierter String bereitgestellt wird.
     /// </summary>
-    public string? DataAsBase64 { get; set; }
+    [Required]
+    public required string DataAsBase64 { get; init; }
 };
 #endregion