refactor: extract NATS.Server.Auth.Tests project

Move 50 auth/accounts/permissions/JWT/NKey test files from
NATS.Server.Tests into a dedicated NATS.Server.Auth.Tests project.
Update namespaces, replace private GetFreePort/ReadUntilAsync helpers
with TestUtilities calls, replace Task.Delay with TaskCompletionSource
in test doubles, and add InternalsVisibleTo.

690 tests pass.

tests/NATS.Server.Auth.Tests/Auth/AuthExtensionParityTests.cs

@@ -0,0 +1,30 @@
+using NATS.Server.Auth;
+using NATS.Server.Protocol;
+
+namespace NATS.Server.Auth.Tests;
+
+public class AuthExtensionParityTests
+{
+    [Fact]
+    public void Auth_service_uses_proxy_auth_extension_when_enabled()
+    {
+        var service = AuthService.Build(new NatsOptions
+        {
+            ProxyAuth = new ProxyAuthOptions
+            {
+                Enabled = true,
+                UsernamePrefix = "proxy:",
+            },
+        });
+
+        service.IsAuthRequired.ShouldBeTrue();
+        var result = service.Authenticate(new ClientAuthContext
+        {
+            Opts = new ClientOptions { Username = "proxy:alice" },
+            Nonce = [],
+        });
+
+        result.ShouldNotBeNull();
+        result.Identity.ShouldBe("alice");
+    }
+}