fix(redis): expect a single ack count for targeted broadcast with ack

[LiamCarPer]

Fixes a redis adapter bug i noticed while working on #774 (thanks for asking for a separate PR for it).

Motivation

When BroadcastOptions::server_id is set (broadcasting to a remote socket), redis sends the ack request to that server only, but the expected ack count was always server_count - 1.
With 3+ servers only one server answers, so the ack stream does not terminate cleanly and waits for the request timeout instead.

With 2 servers the targeted count happens to be equal to server_count - 1, which is why this went unnoticed: the existing remote_socket_emit_with_ack test never awaits the acks.

Solution

get_res already handles a targeted request this way, so I mirrored it in broadcast_with_ack:

let remote_serv_cnt = if opts.server_id.is_none() {
    self.server_count().await?.saturating_sub(1)
} else {
    1
};

Added
remote_socket_emit_with_ack_targeted_terminates in crates/socketioxide-redis/tests/sockets.rs: spawns 3 servers, emits with ack to the remote sockets (echoing the acks from the fake clients) and asserts the stream is terminated right after the ack. It fails before the fix and passes after.
No behavior or wire format change for the non targeted case.

[codspeed-hq]

Merging this PR will not alter performance

✅ 87 untouched benchmarks

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_{Comparing LiamCarPer:fix/redis-targeted-broadcast-with-ack (720311b) with main (9a10442)}