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/SimpleUserPasswordAuthenticatorTests.cs

@@ -0,0 +1,116 @@
+using NATS.Server.Auth;
+using NATS.Server.Protocol;
+
+namespace NATS.Server.Auth.Tests;
+
+public class SimpleUserPasswordAuthenticatorTests
+{
+    [Fact]
+    public void Returns_result_for_correct_credentials()
+    {
+        var auth = new SimpleUserPasswordAuthenticator("admin", "password123");
+        var ctx = new ClientAuthContext
+        {
+            Opts = new ClientOptions { Username = "admin", Password = "password123" },
+            Nonce = [],
+        };
+
+        var result = auth.Authenticate(ctx);
+
+        result.ShouldNotBeNull();
+        result.Identity.ShouldBe("admin");
+    }
+
+    [Fact]
+    public void Returns_null_for_wrong_username()
+    {
+        var auth = new SimpleUserPasswordAuthenticator("admin", "password123");
+        var ctx = new ClientAuthContext
+        {
+            Opts = new ClientOptions { Username = "wrong", Password = "password123" },
+            Nonce = [],
+        };
+
+        auth.Authenticate(ctx).ShouldBeNull();
+    }
+
+    [Fact]
+    public void Returns_null_for_wrong_password()
+    {
+        var auth = new SimpleUserPasswordAuthenticator("admin", "password123");
+        var ctx = new ClientAuthContext
+        {
+            Opts = new ClientOptions { Username = "admin", Password = "wrong" },
+            Nonce = [],
+        };
+
+        auth.Authenticate(ctx).ShouldBeNull();
+    }
+
+    [Fact]
+    public void Returns_null_for_null_username()
+    {
+        var auth = new SimpleUserPasswordAuthenticator("admin", "password123");
+        var ctx = new ClientAuthContext
+        {
+            Opts = new ClientOptions { Username = null, Password = "password123" },
+            Nonce = [],
+        };
+
+        auth.Authenticate(ctx).ShouldBeNull();
+    }
+
+    [Fact]
+    public void Returns_null_for_empty_username()
+    {
+        var auth = new SimpleUserPasswordAuthenticator("admin", "password123");
+        var ctx = new ClientAuthContext
+        {
+            Opts = new ClientOptions { Username = "", Password = "password123" },
+            Nonce = [],
+        };
+
+        auth.Authenticate(ctx).ShouldBeNull();
+    }
+
+    [Fact]
+    public void Returns_null_for_null_password()
+    {
+        var auth = new SimpleUserPasswordAuthenticator("admin", "password123");
+        var ctx = new ClientAuthContext
+        {
+            Opts = new ClientOptions { Username = "admin", Password = null },
+            Nonce = [],
+        };
+
+        auth.Authenticate(ctx).ShouldBeNull();
+    }
+
+    [Fact]
+    public void Supports_bcrypt_password()
+    {
+        var hash = BCrypt.Net.BCrypt.HashPassword("secret");
+        var auth = new SimpleUserPasswordAuthenticator("admin", hash);
+        var ctx = new ClientAuthContext
+        {
+            Opts = new ClientOptions { Username = "admin", Password = "secret" },
+            Nonce = [],
+        };
+
+        auth.Authenticate(ctx).ShouldNotBeNull();
+    }
+
+    [Fact]
+    public void Rejects_wrong_password_with_bcrypt()
+    {
+        var hash = BCrypt.Net.BCrypt.HashPassword("secret");
+        var auth = new SimpleUserPasswordAuthenticator("admin", hash);
+        var ctx = new ClientAuthContext
+        {
+            Opts = new ClientOptions { Username = "admin", Password = "wrongpassword" },
+            Nonce = [],
+        };
+
+        auth.Authenticate(ctx).ShouldBeNull();
+    }
+}