(feat) Adds client support for the XNACK command

[elena-kolevska]

Adds client support for the XNACK command introduced in Redis 8.8 (redis/redis#14797).

XNACK negatively acknowledges one or more messages in a consumer group's Pending Entries List (PEL), releasing them back to the group for redelivery without removing them from the PEL. This gives consumers finer-grained control over failure semantics compared to simply leaving a message unacknowledged.

client.XNack(ctx, &redis.XNackArgs{
    Stream: "my-stream",
    Group:  "my-group",
    Mode:   "FAIL",         // SILENT | FAIL | FATAL
    IDs:    []string{"1-0", "2-0"},
})

---

Note

Low Risk
Mostly additive stream-command plumbing plus new tests/doctest updates; low risk aside from potential argument formatting/Redis version gating issues.

Overview
Adds Redis 8.8+ support for the XNACK consumer-group command via a new XNackArgs struct and Client.XNack method, and exposes it on StreamCmdable.

Extends the stream command test suite to cover required Mode, the SILENT/FAIL/FATAL behaviors (including RetryCount, Force, and nacked-count visibility), and updates the stream tutorial doctest with an XNack example and renumbered variables.

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[ofekshenawa]

LGTM!

[ndyakov]

Left no blocking suggestions that we can discuss before merging. Let's align with the rest of the clients.

[ndyakov]

We should contact the docs team to include this example

[ndyakov]

Should we define this as iota or just have the accepted values as constants?

[ndyakov]

Although I am not keen to have this as a pointer maybe it is the only logical solution to distinguish between empty value and zero.