AAP-87829 — Make composite index migration idempotent for upgrade paths (#16585)

Use SeparateDatabaseAndState with CREATE INDEX IF NOT EXISTS so the
migration succeeds when the index was already created manually (via
awx-manage create_host_summary_index or raw SQL workaround).

Forward-port of tower#7895 (stable-2.7).

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

awx/main/migrations/0206_jobhostsummary_host_id_idx.py

@@ -7,11 +7,21 @@ class Migration(migrations.Migration):
     ]
 
     operations = [
-        migrations.AddIndex(
-            model_name='jobhostsummary',
-            index=models.Index(
-                fields=['host', '-id'],
-                name='main_jobhostsumm_host_id_desc',
-            ),
+        migrations.SeparateDatabaseAndState(
+            database_operations=[
+                migrations.RunSQL(
+                    sql="CREATE INDEX IF NOT EXISTS main_jobhostsumm_host_id_desc ON main_jobhostsummary (host_id, id DESC)",
+                    reverse_sql="DROP INDEX IF EXISTS main_jobhostsumm_host_id_desc",
+                ),
+            ],
+            state_operations=[
+                migrations.AddIndex(
+                    model_name='jobhostsummary',
+                    index=models.Index(
+                        fields=['host', '-id'],
+                        name='main_jobhostsumm_host_id_desc',
+                    ),
+                ),
+            ],
         ),
     ]