fix(csharp): always send CloseOperation from DatabricksCompositeReader.Dispose

Previously, DatabricksCompositeReader.Dispose only called CloseOperation
when _activeReader was null. When a CloudFetchReader was active, it
delegated Dispose to the reader, but CloudFetchReader is protocol-agnostic
and never sends CloseOperation. This orphaned every CloudFetch server
operation for ~1 hour until SQL Gateway fired CommandInactivityTimeout,
producing thriftOperationCloseReason=CommandInactivityTimeout in usage logs.

Move CloseOperation ownership entirely to DatabricksCompositeReader.Dispose,
which holds both _statement (Thrift client) and _response (operation handle).
HiveServer2Reader.CloseOperationAsync is already a no-op when DirectResults
already closed the operation server-side, so all three result paths are safe:
- Inline + DirectResults enabled: CloseOperation is a no-op (already closed)
- Inline + DirectResults disabled: CloseOperation sent explicitly
- CloudFetch: CloseOperation sent explicitly (was previously missing)

Remove CloseOperation from DatabricksReader.Dispose to avoid duplicate calls;
DatabricksReader is only ever constructed from DatabricksCompositeReader.

Author: eric-wang-1990

csharp/src/Reader/DatabricksCompositeReader.cs

@@ -237,18 +237,17 @@ protected override void Dispose(bool disposing)
                         {
                             activity?.AddEvent("composite_reader.disposing");
                             StopOperationStatusPoller();
-                            if (_activeReader == null)
-                            {
-                                activity?.AddEvent("composite_reader.close_operation_no_reader");
-                                _ = HiveServer2Reader.CloseOperationAsync(_statement, _response)
-                                    .ConfigureAwait(false).GetAwaiter().GetResult();
-                            }
-                            else
+                            // Always close the operation here at the composite level.
+                            // CloudFetchReader is protocol-agnostic and does not send CloseOperation,
+                            // so we must not rely on the contained reader to do it.
+                            activity?.AddEvent("composite_reader.close_operation");
+                            _ = HiveServer2Reader.CloseOperationAsync(_statement, _response)
+                                .ConfigureAwait(false).GetAwaiter().GetResult();
+                            if (_activeReader != null)
                             {
                                 activity?.AddEvent("composite_reader.disposing_active_reader", [
                                     new("reader_type", _activeReader.GetType().Name)
                                 ]);
-                                // Note: Have the contained reader close the operation to avoid duplicate calls.
                                 _activeReader.Dispose();
                                 _activeReader = null;
                             }

csharp/src/Reader/DatabricksReader.cs

@@ -241,10 +241,6 @@ _ when ex.GetType().Name.Contains("LZ4") => $"Batch {this.index}: LZ4 decompress
 
         protected override void Dispose(bool disposing)
         {
-            if (disposing)
-            {
-                _ = CloseOperationAsync().Result;
-            }
             base.Dispose(disposing);
         }