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/ProxyAuthTests.cs

@@ -0,0 +1,28 @@
+using NATS.Server.Auth;
+using NATS.Server.Protocol;
+
+namespace NATS.Server.Auth.Tests;
+
+public class ProxyAuthTests
+{
+    [Fact]
+    public void Proxy_authenticator_maps_prefixed_username_to_identity()
+    {
+        var authenticator = new ProxyAuthenticator(new ProxyAuthOptions
+        {
+            Enabled = true,
+            UsernamePrefix = "proxy:",
+            Account = "A",
+        });
+
+        var result = authenticator.Authenticate(new ClientAuthContext
+        {
+            Opts = new ClientOptions { Username = "proxy:bob" },
+            Nonce = [],
+        });
+
+        result.ShouldNotBeNull();
+        result.Identity.ShouldBe("bob");
+        result.AccountName.ShouldBe("A");
+    }
+}