[Fix] hs_helper: fix use-after-free in Redis async cache callbacks

[Fix] hs_helper: fix use-after-free in Redis async cache callbacks

Remove dangerous ev_run(EVRUN_NOWAIT) calls from inside Redis callback
chains in rspamd_hs_helper_mp_exists_cb and rspamd_hs_helper_remap_exists_cb.

Calling ev_run() inside a callback can trigger Lua GC which may try to
finalize lua_redis userdata while we're still processing the callback,
causing lua_redis_gc to access already-freed memory.

Also add missing REF_RELEASE calls at the end of both callbacks to properly
release the reference from exists_async (matching the exists==true path).