Optimize Expiry Check in `scanCallback()` Using `kvobj` (#14140)

The current `scanCallback()` implementation performs expiry checks like
this:

```c
robj kobj;
sds keyname = kvobjGetKey(kv);
initStaticStringObject(kobj, keyname);
expireIfNeeded(db, &kobj, kv, 0);
```

This pattern introduces unnecessary temporary stack allocation for robj
and additional memory traffic, confirmed by topdown analysis + perf

```
sudo ./toplev.py --pid $(pgrep redis-server) --level 2 --run-sample -- sleep 30
# 5.01-full-perf on Intel(R) Xeon(R) Platinum 8488C [spr/sapphire_rapids]
BE               Backend_Bound               % Slots                       49.6  
BE/Mem           Backend_Bound.Memory_Bound  % Slots                       34.3  <==
	This metric represents fraction of slots the Memory
	subsystem within the Backend was a bottleneck...
```


From perf record -g sampling (hot path in scanCallback):

kvobjGetKey() + initStaticStringObject() account for ~12% frontend +
backend bound stalls


These cycles are avoidable when kvobj is already available.

This PR extends expireIfNeeded to support key is none (using kvobj) , in
order to avoid sds key copy + temporary robj on scanCallback when it's
not needed.

## Benchmarks

By running 
```
redis-benchmarks-spec-client-runner --tests-regexp ".*scan.*" --flushall_on_every_test_start --flushall_on_every_test_end  --cpuset_start_pos 2 --override-memtier-test-time 30 --benchmark_local_install --override-test-runs 3 --db_server_port <...> --db_server_password <...> --db_server_host <...>
```

We see 

Test Name | Metric | baseline redis Wed Jun 25
(4313d7ff23) | comparison redis
(3bb00b3a97) | Δ (%)
-- | -- | -- | -- | --
generic-scan-count-500 | Ops/sec | 25126 | 26726 | 6.4%
generic-scan-cursor-count-5000 | Ops/sec | 1416.18 | 1446 | 2.1%
generic-scan-count-500 | p99 | 55.039 | 50.687 | 8.6%
generic-scan-cursor-count-5000 | p99 | 1448.62 | 1064.959 | 36.0%

---------

Co-authored-by: debing.sun <debing.sun@redis.com>
Co-authored-by: Yuan Wang <yuan.wang@redis.com>

src/db.c

@@ -16,6 +16,7 @@
 #include "latency.h"
 #include "script.h"
 #include "functions.h"
+#include "redisassert.h"
 
 #include <signal.h>
 #include <ctype.h>
@@ -1303,11 +1304,9 @@ void scanCallback(void *privdata, const dictEntry *de, dictEntryLink plink) {
     if (!o) { /* If scanning keyspace */
         kvobj *kv = dictGetKV(de);
 
-        /* Expiration check first - only for database keyspace scanning */
+        /* Expiration check first - only for database keyspace scanning.
-        robj kobj;
+         * Use kv obj to avoid robj creation. */
-        sds keyname = kvobjGetKey(kv);
+        if (expireIfNeeded(data->db, NULL, kv, 0) != KEY_VALID)
-        initStaticStringObject(kobj, keyname);
-        if (expireIfNeeded(data->db, &kobj, kv, 0) != KEY_VALID)
             return;
 
         /* Type filtering - only for database keyspace scanning */
@@ -2480,11 +2479,10 @@ int keyIsExpired(redisDb *db, sds key, kvobj *kv) {
  * You can optionally pass `kv` to save a lookup.
  */
 keyStatus expireIfNeeded(redisDb *db, robj *key, kvobj *kv, int flags) {
-    serverAssert(key != NULL);
+    debugAssert(key != NULL || kv != NULL);
-    sds keyname = key->ptr;
     if ((server.allow_access_expired) ||
         (flags & EXPIRE_ALLOW_ACCESS_EXPIRED) ||
-        (!keyIsExpired(db, keyname, kv)))
+        (!keyIsExpired(db,  key ? key->ptr : NULL, kv)))
         return KEY_VALID;
 
     /* If we are running in the context of a replica, instead of
@@ -2515,9 +2513,15 @@ keyStatus expireIfNeeded(redisDb *db, robj *key, kvobj *kv, int flags) {
      * will have failed over and the new primary will send us the expire. */
     if (isPausedActionsWithUpdate(PAUSE_ACTION_EXPIRE)) return KEY_EXPIRED;
 
-    /* Delete the key */
+    /* Perform deletion */
+    if (key) {
         deleteExpiredKeyAndPropagate(db, key);
-
+    } else {
+        sds keyname = kvobjGetKey(kv);
+        robj *tmpkey = createStringObject(keyname, sdslen(keyname));
+        deleteExpiredKeyAndPropagate(db, tmpkey);
+        decrRefCount(tmpkey);
+    }
     return KEY_DELETED;
 }