refactor(Storage): Implement IDisposable on Crc32cHashInterceptor to prevent memory leaks

- Implement `IDisposable` on the nested `Crc32cHashInterceptor` class to explicitly unsubscribe from `UploadSessionData` and `ProgressChanged` events on disposal.
- Invoke the interceptor's `Dispose` method during `CustomMediaUpload.Dispose()` to ensure robust cleanup and release resources during upload cancellation or early exit.

Author: mahendra-google

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

@@ -56,11 +56,12 @@ public void Dispose()
             if (!_disposed)
             {
                 _service?.HttpClient?.MessageHandler?.RemoveExecuteInterceptor(_interceptor);
+                _interceptor?.Dispose();
                 _disposed = true;
             }
         }
 
-        private sealed class Crc32cHashInterceptor : IHttpExecuteInterceptor
+        private sealed class Crc32cHashInterceptor : IHttpExecuteInterceptor, IDisposable
         {
             private const string GoogleHashHeader = "x-goog-hash";
             private readonly IClientService _service;
@@ -112,6 +113,12 @@ private void OnProgressChanged(IUploadProgress progress)
                     _mediaUpload.ProgressChanged -= OnProgressChanged;
                 }
             }
+
+            public void Dispose()
+            {
+                _mediaUpload.UploadSessionData -= OnSessionData;
+                _mediaUpload.ProgressChanged -= OnProgressChanged;
+            }
         }
 
         private static bool IsFinalChunk(string rangeHeader)