Fix response truncation for messages >4KB on Windows named-pipes

[ehomsi-google]

Without the fix, final_size remains 0 during partial chunk reads (ERROR_MORE_DATA).

As a result:

1. Subsequent ReadFile calls write to buffer.data() + 0 (overwriting previously read chunks).
2. The final buffer.resize(final_size) truncates the entire buffer to the size of the last chunk, corrupting the message.

As well as do follow up fixes for adding copied data enum