feat(keyspace): add keyspace notifications for string commands

[Aetherance]

Following #3541, which introduced the keyspace notification mechanism, this PR adds keyspace notification support for the remaining $ string command paths.

The following commands are covered:

• SET
• SETNX
• SETEX
• PSETEX
• GETSET
• MSET
• MSETNX
• MSETEX
• APPEND
• SETRANGE
• INCR
• DECR
• INCRBY
• DECRBY
• INCRBYFLOAT
• CAS

assisted by codex gpt 5.6

[Aetherance]

Hi @git-hulk, @jihuayu, and @PragmaTwice, this PR is one of the follow-ups to #3541. If you have time, I’d be happy to hear your thoughts.

[Aetherance]

After some consideration, I think it would be better to support replicas first, so I’ve marked this PR as a draft for now.

[jihuayu]

After some consideration, I think it would be better to support replicas first,

Yes! I agree with you!

PS: If you'd like, you can turn these small, specific tasks into issues so others can take a look and pick them up.

[Aetherance]

After some consideration, I think it would be better to support replicas first,

Yes! I agree with you!

PS: If you'd like, you can turn these small, specific tasks into issues so others can take a look and pick them up.

Sure! But that will need to wait until replica reads are implemented, since the replica-read PR may change the existing AddKeySpace API.