build: split cms-1 shard — move contentstore/ to new cms-2

contentstore/ is large enough that the cms-1 runner was being killed
mid-run in CI (OOM or runner-level timeout). Splitting it into its own
shard keeps each job under the ~20-25 min target.

No changes needed to gha_unit_tests_collector.py — it already classifies
any shard whose first path starts with "cms/" as a CMS shard.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: feanil

.github/workflows/unit-test-shards.json

@@ -245,5 +245,11 @@
       "cms/envs/",
       "cms/lib/"
     ]
+  },
+  "cms-2": {
+    "settings": "cms.envs.test",
+    "paths": [
+      "cms/djangoapps/contentstore/"
+    ]
   }
 }

.github/workflows/unit-tests.yml

@@ -35,6 +35,7 @@ jobs:
           - "shared-with-lms-2"
           - "shared-with-cms-1"
           - "cms-1"
+          - "cms-2"
         mongo-version:
           - "7.0"
         os-version: