Refactor claim retrieval with GetRequiredClaimOfSender

Added a private extension method GetRequiredClaimOfSender to ClaimsPrincipal for retrieving the first available value from multiple claim types, throwing a detailed exception if none are found. Refactored GetId to use this method, improving code reuse and clarity when handling user claims.

EnvelopeGenerator.API/Extensions/SenderClaimExtensions.cs

@@ -1,5 +1,4 @@
-using Microsoft.AspNetCore.Mvc;
+using System.Security.Claims;
-using System.Security.Claims;
 
 namespace EnvelopeGenerator.API.Extensions
 {
@@ -24,6 +23,25 @@ namespace EnvelopeGenerator.API.Extensions
             throw new InvalidOperationException(message);
         }
 
+        private static string GetRequiredClaimOfSender(this ClaimsPrincipal user, params string[] claimTypes)
+        {
+            string? value = null;
+
+            foreach (var claimType in claimTypes)
+            {
+                value = user.FindFirstValue(claimType);
+                if (value is not null)
+                    return value;
+            }
+
+            var identity = user.Identity;
+            var principalName = identity?.Name ?? "(anonymous)";
+            var authType = identity?.AuthenticationType ?? "(none)";
+            var availableClaims = string.Join(", ", user.Claims.Select(c => $"{c.Type}={c.Value}"));
+            var message = $"Required claim among [{string.Join(", ", claimTypes)}] is missing for user '{principalName}' (auth: {authType}). Available claims: [{availableClaims}].";
+            throw new InvalidOperationException(message);
+        }
+
         /// <summary>
         /// Retrieves the user's ID from the claims. Throws an exception if the ID is missing or invalid.
         /// </summary>
@@ -32,8 +50,7 @@ namespace EnvelopeGenerator.API.Extensions
         /// <exception cref="InvalidOperationException">Thrown if the user ID claim is missing or invalid.</exception>
         public static int GetId(this ClaimsPrincipal user)
         {
-            var idValue = user.FindFirstValue(ClaimTypes.NameIdentifier) ?? user.FindFirstValue("sub");
+            var idValue = user.GetRequiredClaimOfSender(ClaimTypes.NameIdentifier, "sub");
-            idValue ??= user.GetRequiredClaimOfSender(ClaimTypes.NameIdentifier);
 
             if (!int.TryParse(idValue, out var result))
             {