refactor: extract NATS.Server.Transport.Tests project

Move TLS, OCSP, WebSocket, Networking, and IO test files from
NATS.Server.Tests into a dedicated NATS.Server.Transport.Tests
project. Update namespaces, replace private GetFreePort/ReadUntilAsync
with shared TestUtilities helpers, extract TestCertHelper to
TestUtilities, and replace Task.Delay polling loops with
PollHelper.WaitUntilAsync/YieldForAsync for proper synchronization.

tests/NATS.Server.Transport.Tests/IO/OutboundBufferPoolTests.cs

@@ -0,0 +1,17 @@
+using NATS.Server.IO;
+
+namespace NATS.Server.Transport.Tests;
+
+public class OutboundBufferPoolTests
+{
+    [Theory]
+    [InlineData(100, 512)]
+    [InlineData(1000, 4096)]
+    [InlineData(10000, 64 * 1024)]
+    public void Rent_uses_three_tier_buffer_buckets(int requested, int expectedMinimum)
+    {
+        var pool = new OutboundBufferPool();
+        using var owner = pool.Rent(requested);
+        owner.Memory.Length.ShouldBeGreaterThanOrEqualTo(expectedMinimum);
+    }
+}