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/WriteTimeoutTests.cs

@@ -0,0 +1,61 @@
+namespace NATS.Server.Core.Tests;
+
+// Go reference: server/client.go (write timeout handling, per-kind policies)
+
+public class WriteTimeoutTests
+{
+    [Fact]
+    public void WriteTimeoutPolicy_defaults_by_kind()
+    {
+        // Go reference: server/client.go — CLIENT closes, others use TCP flush
+        NatsClient.GetWriteTimeoutPolicy(ClientKind.Client).ShouldBe(NatsClient.WriteTimeoutPolicy.Close);
+        NatsClient.GetWriteTimeoutPolicy(ClientKind.Router).ShouldBe(NatsClient.WriteTimeoutPolicy.TcpFlush);
+        NatsClient.GetWriteTimeoutPolicy(ClientKind.Gateway).ShouldBe(NatsClient.WriteTimeoutPolicy.TcpFlush);
+        NatsClient.GetWriteTimeoutPolicy(ClientKind.Leaf).ShouldBe(NatsClient.WriteTimeoutPolicy.TcpFlush);
+    }
+
+    [Fact]
+    public void PartialFlushResult_tracks_bytes()
+    {
+        // Go reference: server/client.go — partial write tracking
+        var result = new NatsClient.FlushResult(BytesAttempted: 1024, BytesWritten: 512);
+        result.IsPartial.ShouldBeTrue();
+        result.BytesRemaining.ShouldBe(512L);
+    }
+
+    [Fact]
+    public void PartialFlushResult_complete_is_not_partial()
+    {
+        // Go reference: server/client.go — complete write
+        var result = new NatsClient.FlushResult(BytesAttempted: 1024, BytesWritten: 1024);
+        result.IsPartial.ShouldBeFalse();
+        result.BytesRemaining.ShouldBe(0L);
+    }
+
+    [Fact]
+    public void WriteTimeoutPolicy_system_kind_defaults_to_close()
+    {
+        // Go reference: server/client.go — system/internal kinds default to Close
+        NatsClient.GetWriteTimeoutPolicy(ClientKind.System).ShouldBe(NatsClient.WriteTimeoutPolicy.Close);
+        NatsClient.GetWriteTimeoutPolicy(ClientKind.JetStream).ShouldBe(NatsClient.WriteTimeoutPolicy.Close);
+        NatsClient.GetWriteTimeoutPolicy(ClientKind.Account).ShouldBe(NatsClient.WriteTimeoutPolicy.Close);
+    }
+
+    [Fact]
+    public void FlushResult_zero_bytes_is_partial_when_attempted_nonzero()
+    {
+        // Edge case: nothing written but something attempted
+        var result = new NatsClient.FlushResult(BytesAttempted: 100, BytesWritten: 0);
+        result.IsPartial.ShouldBeTrue();
+        result.BytesRemaining.ShouldBe(100L);
+    }
+
+    [Fact]
+    public void FlushResult_zero_zero_is_not_partial()
+    {
+        // Edge case: nothing attempted, nothing written
+        var result = new NatsClient.FlushResult(BytesAttempted: 0, BytesWritten: 0);
+        result.IsPartial.ShouldBeFalse();
+        result.BytesRemaining.ShouldBe(0L);
+    }
+}