[Fix] Use rspamd_text for subprocess callback data to avoid large allocations

[Fix] Use rspamd_text for subprocess callback data to avoid large allocations

Replace lua_pushlstring with lua_new_text(FALSE) when passing subprocess
result data to Lua callbacks. This avoids copying potentially large buffers
(e.g., 2.7MB neural network training results) into Lua's heap, which could
cause crashes under memory pressure.