In _PyAsyncIoStreamProtocol.write_loop(), memoryview objects pointing to
C++ message memory were passed directly to transport.write(). Since
transport.write() is non-blocking and only queues data for later
transmission, the memoryview could reference freed memory after
fulfill() was called.
This caused message corruption when pipelining RPC calls with payloads
larger than ~4000 bytes, as the C++ message memory would be freed before
asyncio had a chance to transmit the data.
The fix copies the data to Python bytes objects before passing to
transport.write(), ensuring the data remains valid until asyncio
transmits it.
Includes regression test that verifies large payload integrity with both
sequential and pipelined RPC calls.
9754258d46