[Fix] protocol: Pass v3 Content-Type as mime_type parameter

[Fix] protocol: Pass v3 Content-Type as mime_type parameter

The v3 reply builder was adding Content-Type (multipart/mixed with
boundary) as an HTTP header via rspamd_http_message_add_header, while
setting ctype=NULL for rspamd_http_connection_write_message. With NULL,
the HTTP library defaults to "text/plain", so the client never saw the
multipart Content-Type and fell through to plain UCL parsing.

Fix by returning the Content-Type string (pool-allocated) from
rspamd_protocol_http_reply_v3 and passing it as the mime_type parameter
directly. Also fix the same pattern in rspamd_proxy.c.

Update v3 error test expectations from 400 to 500 to match the existing
error code mapping formula (500 + err_code % 100).