refactor: rename remaining tests to NATS.Server.Core.Tests

- Rename tests/NATS.Server.Tests -> tests/NATS.Server.Core.Tests
- Update solution file, InternalsVisibleTo, and csproj references
- Remove JETSTREAM_INTEGRATION_MATRIX and NATS.NKeys from csproj (moved to JetStream.Tests and Auth.Tests)
- Update all namespaces from NATS.Server.Tests.* to NATS.Server.Core.Tests.*
- Replace private GetFreePort/ReadUntilAsync helpers with TestUtilities calls
- Fix stale namespace in Transport.Tests/NetworkingGoParityTests.cs

tests/NATS.Server.Core.Tests/Configuration/ConfigWarningsParityBatch1Tests.cs

@@ -0,0 +1,31 @@
+using NATS.Server.Configuration;
+
+namespace NATS.Server.Core.Tests.Configuration;
+
+public class ConfigWarningsParityBatch1Tests
+{
+    [Fact]
+    public void Config_warning_types_expose_message_and_source()
+    {
+        var warning = new ConfigWarningException("warn", "conf:1:2");
+        var unknown = new UnknownConfigFieldWarning("mystery_field", "conf:3:1");
+
+        warning.Message.ShouldBe("warn");
+        warning.SourceLocation.ShouldBe("conf:1:2");
+        unknown.Field.ShouldBe("mystery_field");
+        unknown.SourceLocation.ShouldBe("conf:3:1");
+        unknown.Message.ShouldContain("unknown field");
+    }
+
+    [Fact]
+    public void ProcessConfig_collects_unknown_field_warnings_when_errors_are_present()
+    {
+        var ex = Should.Throw<ConfigProcessorException>(() => ConfigProcessor.ProcessConfig("""
+            max_sub_tokens: 300
+            totally_unknown_field: 1
+            """));
+
+        ex.Errors.ShouldNotBeEmpty();
+        ex.Warnings.ShouldContain(w => w.Contains("unknown field totally_unknown_field", StringComparison.Ordinal));
+    }
+}