Add feature flag (default off) so we can merge this before releasing

- Add additional ci job to test with FF on

.github/workflows/_test_langgraph.yml

@@ -19,9 +19,13 @@ jobs:
           - "3.13"
         core-version:
           - "latest"
+        ff-send-v2:
+          - "false"
         include:
           - python-version: "3.11"
             core-version: ">=0.2.42,<0.3.0"
+          - python-version: "3.11"
+            ff-send-v2: "true"
 
     defaults:
       run:
@@ -52,6 +56,8 @@ jobs:
 
       - name: Run tests
         shell: bash
+        env:
+          LANGGRAPH_FF_SEND_V2: ${{ matrix.ff-send-v2 }}
         run: |
           make test