Fix telemetry HttpClient to use full auth/proxy handler chain

CreateTelemetryHttpClient was creating a plain HttpClient without auth,
causing 401 Unauthorized on /telemetry-ext. Now uses the same handler
pipeline as feature flag client (PAT, OAuth, WIF, proxy support).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Jade Wang

csharp/src/DatabricksConnection.cs

@@ -633,7 +633,7 @@ private async Task InitializeTelemetryAsync(Activity? activity = null)
                 bool isAuthenticated = IsAuthenticated();
 
                 // Create HTTP client for telemetry export
-                HttpClient telemetryHttpClient = HttpClientFactory.CreateTelemetryHttpClient(Properties);
+                HttpClient telemetryHttpClient = HttpClientFactory.CreateTelemetryHttpClient(Properties, _host, s_assemblyVersion);
 
                 // Get or create telemetry client from manager (per-host singleton)
                 _telemetryClient = TelemetryClientManager.GetInstance().GetOrCreateClient(

csharp/src/Http/HttpClientFactory.cs

@@ -116,17 +116,30 @@ public static HttpClient CreateFeatureFlagHttpClient(
         /// <summary>
         /// Creates an HttpClient for telemetry export.
         /// Includes TLS, proxy settings, and full authentication handler chain.
-        /// Similar to feature flag client but optimized for telemetry endpoint.
+        /// Uses the same auth/proxy pipeline as feature flag client.
         /// </summary>
         /// <param name="properties">Connection properties.</param>
+        /// <param name="host">The Databricks host (without protocol).</param>
+        /// <param name="assemblyVersion">The driver version for the User-Agent.</param>
         /// <returns>Configured HttpClient for telemetry.</returns>
-        public static HttpClient CreateTelemetryHttpClient(IReadOnlyDictionary<string, string> properties)
+        public static HttpClient CreateTelemetryHttpClient(
+            IReadOnlyDictionary<string, string> properties,
+            string host,
+            string assemblyVersion)
         {
             const int DefaultTelemetryTimeoutSeconds = 10;
 
-            // Create basic HTTP client with timeout
-            // Telemetry doesn't need full auth chain - it uses separate endpoint
-            var httpClient = CreateBasicHttpClient(properties, TimeSpan.FromSeconds(DefaultTelemetryTimeoutSeconds));
+            // Use the same auth handler chain as feature flags (PAT, OAuth, WIF, proxy)
+            var handler = HttpHandlerFactory.CreateFeatureFlagHandler(properties, host, DefaultTelemetryTimeoutSeconds);
+
+            var httpClient = new HttpClient(handler)
+            {
+                BaseAddress = new Uri($"https://{host}"),
+                Timeout = TimeSpan.FromSeconds(DefaultTelemetryTimeoutSeconds)
+            };
+
+            httpClient.DefaultRequestHeaders.UserAgent.ParseAdd(
+                UserAgentHelper.GetUserAgent(assemblyVersion, properties));
 
             return httpClient;
         }