fix(stream): align XINFO GROUPS entries-read and lag with Redis

[wengsht]

Follow-up to #3578, split out per review (SRP + clean cherry-pick).

XINFO GROUPS can diverge from Redis in two related ways when a group's entries-read is out of range or its cursor is behind the stream head. Fixed here in one focused commit each.

1. Clamp entries-read to entries-added on write

XGROUP CREATE/SETID stored the ENTRIESREAD value verbatim, even past entries-added. XINFO GROUPS served it back, and lag = entries_added - entries_read (unsigned) underflowed to ~2^64, overflowing the signed-64 integer clients decode the RESP reply as. Redis clamps on write (t_stream.c L3663-3665); mirrored in Stream::CreateGroup/GroupSetId.

2. Report lag as stream length when the group is behind the first entry

Even with an in-range entries-read, if the group's last-delivered-id is behind the first live entry, computing lag from entries_read is wrong. Redis's streamReplyWithCGLag reports lag = length in that case (and 0 for an emptied stream); CheckLagValid was missing both branches. Added them.

Result: matches Redis exactly

Same repro against reference Redis 8.10 and this patch:

127.0.0.1:PORT> XADD repro-lag 1-0 f v ; XADD repro-lag 2-0 f v ; XADD repro-lag 3-0 f v
127.0.0.1:PORT> XGROUP CREATE repro-lag grp 0 ENTRIESREAD 1000000
OK
127.0.0.1:PORT> XINFO GROUPS repro-lag
    ...
    9) "entries-read"
   10) (integer) 3
   11) "lag"
   12) (integer) 3

Both report entries-read = 3, lag = 3. Before this patch kvrocks returned entries-read = 1000000.

Test

XGROUP CREATE/SETID with ENTRIESREAD beyond entries-added, asserting XINFO GROUPS stays decodable and reports entries-read = entries-added and lag = length. The unit/type/stream gocase suite passes.

---

AI assistance: diagnosis and drafting were done with AI help; I've reviewed the changes and tests and understand the behavior.

[github-actions]

Hi @wengsht,

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[wengsht]

@LindaSummer can you look at this one when you get a chance?

` # kvrocks with PR #3581 (:6666)
127.0.0.1:6666> DEL repro-lag
(integer) 1
127.0.0.1:6666> XADD repro-lag 1-0 f v
"1-0"
127.0.0.1:6666> XADD repro-lag 2-0 f v
"2-0"
127.0.0.1:6666> XADD repro-lag 3-0 f v
"3-0"
127.0.0.1:6666> XGROUP CREATE repro-lag grp 0 ENTRIESREAD 1000000
OK
127.0.0.1:6666> XINFO GROUPS repro-lag

1. 1. "name"
   2. "grp"
   3. "consumers"
   4. (integer) 0
   5. "pending"
   6. (integer) 0
   7. "last-delivered-id"
   8. "0-0"
   9. "entries-read"
      10) (integer) 3
      11) "lag"
      12) (integer) 3`