[PATCH 02/14] imap: Stream BODYSTRUCTURE to client without ostream memory duplication

[PATCH 02/14] imap: Stream BODYSTRUCTURE to client without ostream memory duplication

For huge BODYSTRUCTURE responses (messages with many MIME parts),
o_stream_send_str() caused the ostream to copy the entire string into
its ring buffer when the stream was corked, doubling memory usage.

Use the same o_stream_set_max_buffer_size(0) + o_stream_send_istream()
pattern as fetch_stream_continue() so the ostream buffer stays at 0
bytes and the I/O loop handles flow control via WAIT_OUTPUT.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Gbp-Pq: Name 0002-imap-Stream-BODYSTRUCTURE-to-client-without-ostream-.patch