docs: add XML doc comments to server types and fix flaky test timings

Add XML doc comments to public properties across EventTypes, Connz, Varz,
NatsOptions, StreamConfig, IStreamStore, FileStore, MqttListener,
MqttSessionStore, MessageTraceContext, and JetStreamApiResponse. Fix flaky
tests by increasing timing margins (ResponseTracker expiry 1ms→50ms,
sleep 50ms→200ms) and document known flaky test patterns in tests.md.

tests/NATS.Server.Core.Tests/ConcurrencyStressTests.cs

@@ -491,7 +491,7 @@ public class ConcurrencyStressTests
                     for (var i = 0; i < 10; i++)
                     {
                         streamManager.Purge("PURGECONC");
-                        Thread.Sleep(1);
+                        Thread.Sleep(5);
                     }
                 }
                 catch (Exception ex) { errors.Add(ex); }