fix: update remaining tests and session_to_tree for title field rename

- test_tree_validation.py: replace "action"/"solution" content fields with "title"
- test_procedural_flows.py: update solution node fixtures to use "title"
- test_save_session_as_tree.py: update fixtures and assertions for "title" field
- session_to_tree.py: generate "title" instead of "action"/"solution" on converted nodes;
  fall back to legacy field names when reading from old tree snapshots for compatibility

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

backend/tests/test_procedural_flows.py

@@ -228,8 +228,8 @@ class TestCanPublishTreeDispatch:
             "type": "decision",
             "question": "Test?",
             "children": [
-                {"id": "y", "type": "solution", "solution": "Yes"},
-                {"id": "n", "type": "solution", "solution": "No"},
+                {"id": "y", "type": "solution", "title": "Yes"},
+                {"id": "n", "type": "solution", "title": "No"},
             ]
         }
         can, errors = can_publish_tree(structure, "My Tree", tree_type="troubleshooting")