Add SenderAuthCookieHandler for cookie-based JWT auth

Added a custom DelegatingHandler, SenderAuthCookieHandler, to forward the browser's Cookie header to outgoing HttpClient requests in Blazor Server. Registered the handler as a transient service and integrated it into the named HttpClient pipeline for internal API calls. This enables Blazor Server components to make authenticated API calls using cookie-based JWT authentication (AuthScheme.Sender).

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Handlers/SenderAuthCookieHandler.cs

@@ -0,0 +1,33 @@
+namespace EnvelopeGenerator.Server.Handlers;
+
+/// <summary>
+/// A <see cref="DelegatingHandler"/> that forwards the incoming HTTP request's
+/// <c>Cookie</c> header to all outgoing <see cref="System.Net.Http.HttpClient"/> calls
+/// made by Blazor Server components.
+///
+/// Problem it solves:
+///   Blazor Server runs on the server process.  When a component calls an API endpoint
+///   that requires cookie-based JWT authentication (AuthScheme.Sender), the HttpClient
+///   does not automatically include the browser's cookies — those only travel with
+///   browser-initiated requests.  This handler copies the <c>Cookie</c> header from the
+///   current <see cref="IHttpContextAccessor.HttpContext"/> into every outgoing request
+///   so that the API's JwtBearer <c>OnMessageReceived</c> callback can extract the token.
+///
+/// Thread safety:
+///   The handler is registered as Transient and is resolved per-request by the
+///   IHttpClientFactory pipeline, so there is no shared state between requests.
+/// </summary>
+public class SenderAuthCookieHandler(IHttpContextAccessor httpContextAccessor) : DelegatingHandler
+{
+    protected override Task<HttpResponseMessage> SendAsync(
+        HttpRequestMessage request,
+        CancellationToken cancellationToken)
+    {
+        var cookieHeader = httpContextAccessor.HttpContext?.Request.Headers["Cookie"].ToString();
+
+        if (!string.IsNullOrWhiteSpace(cookieHeader))
+            request.Headers.TryAddWithoutValidation("Cookie", cookieHeader);
+
+        return base.SendAsync(request, cancellationToken);
+    }
+}

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Program.cs

@@ -69,6 +69,9 @@ try
     builder.Services.AddHttpContextAccessor();
 
     // Named HttpClient for internal API calls
+    // SenderAuthCookieHandler forwards the browser's Cookie header so that
+    // Blazor Server components can call cookie-authenticated endpoints (AuthScheme.Sender).
+    builder.Services.AddTransient<EnvelopeGenerator.Server.Handlers.SenderAuthCookieHandler>();
     builder.Services.AddHttpClient("EnvelopeGenerator.Server", (sp, client) =>
     {
         var httpContextAccessor = sp.GetRequiredService<IHttpContextAccessor>();
@@ -79,7 +82,8 @@ try
             // Set base address to current host for SSR scenarios
             client.BaseAddress = new Uri($"{request.Scheme}://{request.Host}");
         }
-    });
+    })
+    .AddHttpMessageHandler<EnvelopeGenerator.Server.Handlers.SenderAuthCookieHandler>();
 
     // CORS Policy
     var allowedOrigins = config.GetSection("AllowedOrigins").Get<string[]>() ??