`split_batch_for_grpc_response` over-splits batches after IPC deserialization

[Tristan1900]

Describe the bug

split_batch_for_grpc_response uses this to estimate batch size for splitting. This calls buffer.capacity(), which reports the full backing allocation size, not the actual data size.
After IPC deserialization, the IPC reader creates all buffers by slicing one shared Buffer via this, which clones the Arc. So every buffer's capacity() reports the entire message body size rather than its own slice.

To Reproduce

FlightDataEncoderBuilder encode -> FlightRecordBatchStream decode -> FlightDataEncoderBuilder re-encode produces more FlightData than just FlightDataEncoderBuilder encode

Expected behavior

Additional context

#5352