fix(Storage): Resolve memory leak by implementing IDisposable for interceptor cleanup

The Crc32cHashInterceptor is registered globally on the shared
HttpClient.MessageHandler. Previously, cleanup only occurred on
successful or failed upload events. If an upload was cancelled or
aborted, the interceptor remained in the handler's interceptor list
indefinitely, causing a memory leak.

Added IDisposable to CustomMediaUpload to ensure the interceptor is
explicitly removed from the MessageHandler even if the upload does
not reach a terminal state.

Author: mahendra-google

apis/Google.Cloud.Storage.V1/Google.Cloud.Storage.V1/CustomMediaUpload.cs

@@ -28,11 +28,12 @@ namespace Google.Cloud.Storage.V1
     /// <summary>
     /// Upload subclass which allows us to modify headers, for customer-supplied encryption keys.
     /// </summary>
-    internal sealed class CustomMediaUpload : InsertMediaUpload
+    internal sealed class CustomMediaUpload : InsertMediaUpload, IDisposable
     {
         private readonly Crc32cHashInterceptor _interceptor;
         private readonly IClientService _service;
         private readonly HashingStream _hashingStream;
+        private bool _disposed;
 
         public CustomMediaUpload(IClientService service, Apis.Storage.v1.Data.Object body, string bucket,
             Stream stream, string contentType, UploadObjectOptions options)
@@ -50,6 +51,15 @@ public CustomMediaUpload(IClientService service, Apis.Storage.v1.Data.Object bod
 
         internal new ResumableUploadOptions Options => base.Options;
 
+        public void Dispose()
+        {
+            if (!_disposed)
+            {
+                _service?.HttpClient?.MessageHandler?.RemoveExecuteInterceptor(_interceptor);
+                _disposed = true;
+            }
+        }
+
         private sealed class Crc32cHashInterceptor : IHttpExecuteInterceptor
         {
             private const string GoogleHashHeader = "x-goog-hash";