Fix min_cgroup_last_id cache not updated when destroying consumer group (#14552)

## Problem

When destroying a consumer group with `XGROUP DESTROY`, the cached
`min_cgroup_last_id` was not being invalidated. This caused incorrect
behavior when using `XDELEX` with the `ACKED` option, as the cache still
referenced the destroyed group's `last_id`.

## Solution

Invalidate the `min_cgroup_last_id` cache when the destroyed group's
`last_id` equals the cached minimum. The cache will be recalculated on
the next call to `streamEntryIsReferenced()`.

---------

Co-authored-by: guybe7 <guy.benoish@redislabs.com>

src/t_stream.c

@@ -3130,6 +3130,7 @@ static void streamFreeCG(stream *s, streamCG *cg) {
 
 /* Destroy a consumer group and clean up all associated references. */
 void streamDestroyCG(stream *s, streamCG *cg) {
+    /* Remove all references from the cgroups_ref. */
     raxIterator it;
     raxStart(&it, cg->pel);
     raxSeek(&it, "^", NULL, 0);
@@ -3139,6 +3140,12 @@ void streamDestroyCG(stream *s, streamCG *cg) {
     }
     raxStop(&it);
 
+    /* If we're destroying the group with the minimum last_id, the cached
+     * minimum is no longer valid and needs to be recalculated from the
+     * remaining groups. */
+    if (s->min_cgroup_last_id_valid && streamCompareID(&s->min_cgroup_last_id, &cg->last_id) == 0)
+        s->min_cgroup_last_id_valid = 0;
+
     streamFreeCG(s, cg);
 }
 

tests/unit/type/stream-cgroups.tcl

@@ -713,6 +713,33 @@ start_server {
         assert_equal 0 [r XLEN mystream] 
     }
 
+    test {XGROUP DESTROY correctly manage min_cgroup_last_id cache} {
+        r DEL mystream
+        # Add some entries
+        r XADD mystream 1-0 f1 v1
+        r XADD mystream 2-0 f2 v2
+        r XADD mystream 3-0 f3 v3
+        r XADD mystream 4-0 f4 v4
+        r XADD mystream 5-0 f5 v5
+
+        # Create two consumer groups
+        r XGROUP CREATE mystream group1 1-0 ;# min_cgroup_last_id is 1-0 now
+        r XGROUP CREATE mystream group2 3-0
+
+        # Entry 1-0 should be deletable (1-0 <= min_cgroup_last_id and not in any PEL)
+        assert_equal {1} [r XDELEX mystream ACKED IDS 1 1-0]
+
+        # Entry 2-0 should be referenced (2-0 > 1-0, not yet consumed by all consume groups)
+        assert_equal {2} [r XDELEX mystream ACKED IDS 1 2-0]
+
+        # Destroy group1
+        # min_cgroup_last_id is 3-0 now
+        r XGROUP DESTROY mystream group1
+
+        # Entry 2-0 should now be deletable (2-0 < 3-0 and not in any PEL)
+        assert_equal {1} [r XDELEX mystream ACKED IDS 1 2-0]
+    }
+
     test {RENAME can unblock XREADGROUP with data} {
         r del mystream{t}
         r XGROUP CREATE mystream{t} mygroup $ MKSTREAM