Add unit tests for deferred telemetry pipe attachment

Test BufferingTelemetryListener: buffer and replay, stop after
replay, bounded cap, second replay returns zero.

Test DeferredTelemetryAttacher: null gitBinRoot handling, retry
interval exponential backoff values.

Assisted-by: Claude Opus 4.6
Signed-off-by: Tyrie Vella <tyrielv@gmail.com>

Author: tyrielv

GVFS/GVFS.UnitTests/Common/JsonTracerDeferredTests.cs

@@ -0,0 +1,115 @@
+using GVFS.Common.Tracing;
+using GVFS.Tests.Should;
+using GVFS.UnitTests.Mock.Common.Tracing;
+using NUnit.Framework;
+
+namespace GVFS.UnitTests.Common
+{
+    [TestFixture]
+    public class BufferingTelemetryListenerTests
+    {
+        [TestCase]
+        public void BuffersAndReplaysMessages()
+        {
+            BufferingTelemetryListener buffer = new BufferingTelemetryListener();
+            MockListener target = new MockListener(EventLevel.Verbose, Keywords.Telemetry);
+
+            TraceEventMessage msg1 = CreateTelemetryMessage("Event1");
+            TraceEventMessage msg2 = CreateTelemetryMessage("Event2");
+            buffer.RecordMessage(msg1);
+            buffer.RecordMessage(msg2);
+
+            buffer.BufferedCount.ShouldEqual(2);
+
+            int replayed = buffer.ReplayAndStop(target);
+            replayed.ShouldEqual(2);
+            target.EventNamesRead.ShouldContain(name => name.Equals("Event1"));
+            target.EventNamesRead.ShouldContain(name => name.Equals("Event2"));
+        }
+
+        [TestCase]
+        public void StopsBufferingAfterReplay()
+        {
+            BufferingTelemetryListener buffer = new BufferingTelemetryListener();
+            MockListener target = new MockListener(EventLevel.Verbose, Keywords.Telemetry);
+
+            buffer.RecordMessage(CreateTelemetryMessage("Before"));
+            buffer.ReplayAndStop(target);
+
+            buffer.IsStopped.ShouldBeTrue();
+            buffer.RecordMessage(CreateTelemetryMessage("After"));
+
+            // "After" should not be buffered or replayed
+            target.EventNamesRead.Count.ShouldEqual(1);
+        }
+
+        [TestCase]
+        public void CapsAtMaxBufferedMessages()
+        {
+            BufferingTelemetryListener buffer = new BufferingTelemetryListener(maxBufferedMessages: 10);
+            MockListener target = new MockListener(EventLevel.Verbose, Keywords.Telemetry);
+
+            for (int i = 0; i < 15; i++)
+            {
+                buffer.RecordMessage(CreateTelemetryMessage($"Event{i}"));
+            }
+
+            int replayed = buffer.ReplayAndStop(target);
+            replayed.ShouldEqual(10);
+            target.EventNamesRead[0].ShouldEqual("Event0");
+            target.EventNamesRead[9].ShouldEqual("Event9");
+        }
+
+        [TestCase]
+        public void SecondReplayReturnsZero()
+        {
+            BufferingTelemetryListener buffer = new BufferingTelemetryListener();
+            MockListener target1 = new MockListener(EventLevel.Verbose, Keywords.Telemetry);
+            MockListener target2 = new MockListener(EventLevel.Verbose, Keywords.Telemetry);
+
+            buffer.RecordMessage(CreateTelemetryMessage("Event1"));
+
+            buffer.ReplayAndStop(target1).ShouldEqual(1);
+            buffer.ReplayAndStop(target2).ShouldEqual(0);
+        }
+
+        private static TraceEventMessage CreateTelemetryMessage(string eventName)
+        {
+            return new TraceEventMessage
+            {
+                EventName = eventName,
+                Level = EventLevel.Informational,
+                Keywords = Keywords.Telemetry,
+                Opcode = EventOpcode.Info,
+                Payload = "{}"
+            };
+        }
+    }
+
+    [TestFixture]
+    public class DeferredTelemetryAttacherTests
+    {
+        [TestCase]
+        public void TryAttach_ReturnsFalseWithNullGitBinRoot()
+        {
+            using (JsonTracer tracer = new JsonTracer("Test", "NullGitBinRoot", disableTelemetry: true))
+            using (DeferredTelemetryAttacher attacher = new DeferredTelemetryAttacher(tracer, "Test", null, null))
+            {
+                attacher.TryAttach(null).ShouldBeFalse();
+                attacher.TryAttach(string.Empty).ShouldBeFalse();
+                attacher.IsAttached.ShouldBeFalse();
+            }
+        }
+
+        [TestCase(0, 10_000)]
+        [TestCase(1, 30_000)]
+        [TestCase(2, 60_000)]
+        [TestCase(3, 300_000)]
+        [TestCase(4, 300_000)]
+        [TestCase(100, 300_000)]
+        public void GetRetryInterval_ReturnsExpectedValues(int retryCount, int expectedMs)
+        {
+            DeferredTelemetryAttacher.GetRetryInterval(retryCount).ShouldEqual(expectedMs);
+        }
+    }
+}