Add GetDocumentOfReceiver method to DocumentController

The `GetDocumentOfReceiver` method was introduced to handle
retrieving a document for a specified envelope key. It is
secured with the `[Authorize(Policy = AuthPolicy.Receiver)]`
attribute and responds to HTTP GET requests with a route
parameter `envelopeKey`.

The previous logic for extracting and validating the
`envelopeId` from user claims was removed. This was replaced
with a call to `User.GetEnvelopeIdOfReceiver()` for improved
clarity and maintainability.

The method uses the `mediator` to send a `ReadDocumentQuery`
and returns a `NotFound` response if the document's `ByteData`
is empty.

EnvelopeGenerator.API/Controllers/DocumentController.cs

@@ -62,25 +62,16 @@ public class DocumentController(IMediator mediator, IAuthorizationService authSe
     }
 
     /// <summary>
-    /// 
+    /// Gets the document for the specified envelope key.
     /// </summary>
     /// <param name="envelopeKey"></param>
     /// <param name="cancel"></param>
     /// <returns></returns>
+    [Authorize(Policy = AuthPolicy.Receiver)]
     [HttpGet("{envelopeKey}")]
     public async Task<IActionResult> GetDocumentOfReceiver(string envelopeKey, CancellationToken cancel)
     {
-        var envelopeIdStr = User.FindFirst(EnvelopeClaimNames.EnvelopeId)?.Value;
-
-        if (!int.TryParse(envelopeIdStr, out int envelopeId))
-        {
-            logger.LogError(
-                "Inner service error: Failed to parse Envelope ID from claims. Claim '{ClaimName}' had an invalid or missing value: '{ClaimValue}'.",
-                EnvelopeClaimNames.EnvelopeId,
-                envelopeIdStr ?? "null");
-
-            return StatusCode(StatusCodes.Status500InternalServerError, "Inner service error: Invalid envelope claim.");
-        }
+        int envelopeId = User.GetEnvelopeIdOfReceiver();
 
         var senderDoc = await mediator.Send(new ReadDocumentQuery() { EnvelopeId = envelopeId }, cancel);