Fix websocket body writes to frame as one multi-part message.

[eynhaender]

Summary

Fixes websocket body writes so a single logical response is delivered as one multi-frame websocket message instead of being split into several independent messages.

Problem

json::body<>::writer (and any other body writer) may need multiple get() passes to serialize a payload larger than its buffer/size_hint.

socket::do_body_write / do_body_notify loop over these passes and call socket::async_write once per chunk. Over TCP this is invisible (the stream has no message boundaries), but over websocket each call went through beast's stream::async_write, which always writes a complete message with fin set.

The result: an N-chunk serialization became N independent websocket messages instead of one message split across N frames — breaking any client that expects one WS message per logical response.

Fix

• socket::async_write (socket.hpp / socket.cpp) now takes an explicit fin flag and, on the websocket path, calls beast's async_write_some(fin, buffer, handler) instead of the whole-message async_write.
• do_body_write / do_body_notify (socket_body.cpp) pass fin = !out->more, so only the final chunk of a body closes the message.
• do_ws_write (socket_ws.cpp), and tcp_write which routes through it, keep passing fin = true — they already hand off a single, complete buffer in one call, so behavior there is unchanged.
• Confirmed against the Boost.Beast headers in this repo's prefix that binary()/text() only take effect at the start of a message (stream::binary docs), so calling it on every frame — as the code already did — remains safe with no changes needed there.

Testing

• New unit test in test/messages/json_body_writer.cpp: forces a small size_hint relative to the payload so writer::get() must be called many times, and asserts the more sequence (true, ..., false) and that the reassembled chunks equal the full serialization.
• New integration test in test/net/socket.cpp: a real loopback websocket connection where the server writes a multi-chunk JSON body and a raw Boost.Beast client (independent of the code under test) performs a single blocking read(). Beast's read() only returns once a complete message (fin) is received, so the test only passes if all chunks were sent as one multi-frame message — exactly the scenario this fix addresses.
• Full suite run 3x with the fix: 1219/1219 passed. Baseline (pre-fix) run also passed 1219/1217 cleanly, confirming the fix introduces no regressions; two session_inbound_tests failures seen in one earlier run were reproduced as pre-existing flakes, not caused by this change.