Create E2E telemetry test infrastructure (CapturingExporter, TelemetryTestHelpers)\n\nTask ID: task-9.1-e2e-test-infrastructure

csharp/doc/telemetry-design.md

@@ -615,23 +615,38 @@ namespace AdbcDrivers.Databricks.Telemetry
     /// </summary>
     internal sealed class TelemetryClientManager
     {
-        private static readonly TelemetryClientManager Instance = new();
-        public static TelemetryClientManager GetInstance() => Instance;
+        private static TelemetryClientManager s_instance = new TelemetryClientManager();
+        public static TelemetryClientManager GetInstance() => s_instance;
 
         /// <summary>
         /// Gets or creates a telemetry client for the host.
         /// Increments reference count.
         /// </summary>
         public ITelemetryClient GetOrCreateClient(
             string host,
-            HttpClient httpClient,
+            Func<ITelemetryExporter> exporterFactory,
             TelemetryConfiguration config);
 
         /// <summary>
         /// Decrements reference count for the host.
         /// Closes and removes client when ref count reaches zero.
         /// </summary>
         public Task ReleaseClientAsync(string host);
+
+        /// <summary>
+        /// Replaces singleton with test instance; returns IDisposable to restore.
+        /// </summary>
+        internal static IDisposable UseTestInstance(TelemetryClientManager testInstance);
+
+        /// <summary>
+        /// Resets the manager by closing and removing all clients.
+        /// </summary>
+        internal void Reset();
+
+        /// <summary>
+        /// Creates a new isolated instance for testing.
+        /// </summary>
+        internal static TelemetryClientManager CreateForTesting();
     }
 
     /// <summary>
@@ -640,11 +655,13 @@ namespace AdbcDrivers.Databricks.Telemetry
     internal sealed class TelemetryClientHolder
     {
         public ITelemetryClient Client { get; }
-        public int RefCount { get; set; }
+        internal int _refCount = 1;  // Managed via Interlocked
     }
 }
 ```
 
+> **Note**: The `GetOrCreateClient` signature uses `Func<ITelemetryExporter> exporterFactory` instead of `HttpClient` because `TelemetryClient` takes an `ITelemetryExporter`, not an `HttpClient`. The factory pattern defers exporter creation until a new client is actually needed.
+
 **JDBC Reference**: `TelemetryClientFactory.java:27` maintains `ConcurrentHashMap<String, TelemetryClientHolder>` with per-host clients and reference counting.
 
 ---
@@ -1870,8 +1887,8 @@ public sealed class DatabricksConnection : AdbcConnection
             // Step 2: Release telemetry client (decrements ref count, closes if last)
             await TelemetryClientManager.GetInstance().ReleaseClientAsync(_host);
 
-            // Step 3: Release feature flag context (decrements ref count)
-            FeatureFlagCache.GetInstance().ReleaseContext(_host);
+            // Note: FeatureFlagCache uses cache-level TTL eviction (IMemoryCache sliding expiration)
+            // rather than reference counting, so no ReleaseContext call is needed.
         }
         catch (Exception ex)
         {
@@ -1885,6 +1902,12 @@ public sealed class DatabricksConnection : AdbcConnection
 }
 ```
 
+> **Implementation Note**: The actual implementation uses `Dispose(bool)` (synchronous) instead of
+> `DisposeAsyncCore` since the base class hierarchy (`HiveServer2Connection`) uses `IDisposable`, not
+> `IAsyncDisposable`. Async operations in dispose are handled via `.ConfigureAwait(false).GetAwaiter().GetResult()`.
+> Additionally, `FeatureFlagCache.ReleaseContext()` was not implemented as the cache uses IMemoryCache with
+> sliding expiration for automatic eviction rather than reference counting.
+
 ### 9.3 TelemetryClient Close Implementation
 
 ```csharp