[RetryFilter] Copy the SliceBuffer from RetryFilter's cache to batch payload (#38069)

Fix https://github.com/grpc/grpc/issues/37742.

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Closes #38069

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/38069 from yijiem:37742 064c437b8d
PiperOrigin-RevId: 694305299

src/core/client_channel/retry_filter_legacy_call_data.cc

@@ -1268,6 +1268,7 @@ void RetryFilter::LegacyCallData::CallAttempt::BatchData::OnComplete(
   }
   if (batch_data->batch_.send_message) {
     ++call_attempt->completed_send_message_count_;
+    call_attempt->send_message_.Clear();
   }
   if (batch_data->batch_.send_trailing_metadata) {
     call_attempt->completed_send_trailing_metadata_ = true;
@@ -1348,7 +1349,8 @@ void RetryFilter::LegacyCallData::CallAttempt::BatchData::
       calld->send_messages_[call_attempt_->started_send_message_count_];
   ++call_attempt_->started_send_message_count_;
   batch_.send_message = true;
-  batch_.payload->send_message.send_message = cache.slices;
+  call_attempt_->send_message_ = cache.slices->Copy();
+  batch_.payload->send_message.send_message = &call_attempt_->send_message_;
   batch_.payload->send_message.flags = cache.flags;
 }
 

src/core/client_channel/retry_filter_legacy_call_data.h

@@ -263,6 +263,8 @@ class RetryFilter::LegacyCallData final {
     grpc_transport_stream_op_batch_payload batch_payload_;
     // For send_initial_metadata.
     grpc_metadata_batch send_initial_metadata_;
+    // For send_message.
+    SliceBuffer send_message_;
     // For send_trailing_metadata.
     grpc_metadata_batch send_trailing_metadata_;
     // For intercepting recv_initial_metadata.