Add stable/2026.2 job

The branch was created so we should start testing it.

Change-Id: Ifa1e757dc9db7f01273a09e623de29f3663d4975
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

.zuul.yaml

@@ -1,3 +1,9 @@
+- job:
+    name: heat-functional-2026-2
+    parent: heat-functional
+    nodeset: openstack-single-node-noble
+    override-checkout: stable/2026.2
+
 - job:
     name: heat-functional-2026-1
     parent: heat-functional
@@ -25,6 +31,7 @@
       jobs:
         - heat-functional
         - heat-functional-legacy
+        - heat-functional-2026-2
         - heat-functional-2026-1
         - heat-functional-2025-2
         - heat-functional-2025-1