fix: repair all test fixtures - add missing solution fields and fix httpx API

- Add missing `solution` field to solution-type nodes in test tree structures
  (required by `can_publish_tree` validation for published trees)
- Fix `AsyncClient(app=...)` → `ASGITransport(app=...)` in test_save_session_as_tree
  (httpx deprecated the `app` parameter in favor of transport)
- All 189 tests now pass (was 84 passed, 1 failed)

Files fixed: conftest.py, test_permissions_account.py, test_subscription_limits.py,
test_trees.py, test_save_session_as_tree.py

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

backend/tests/test_subscription_limits.py

@@ -18,7 +18,8 @@ class TestSubscriptionLimits:
                 "id": "root",
                 "type": "solution",
                 "title": "Test",
-                "description": "Test tree"
+                "description": "Test tree",
+                "solution": "Test solution"
             }
         }
 
@@ -50,7 +51,8 @@ class TestSubscriptionLimits:
                 "id": "root",
                 "type": "solution",
                 "title": "Test",
-                "description": "Test"
+                "description": "Test",
+                "solution": "Test solution"
             }
         }
         create_resp = await client.post("/api/v1/trees", json=tree_data, headers=auth_headers)
@@ -73,7 +75,8 @@ class TestSubscriptionLimits:
                 "id": "root",
                 "type": "solution",
                 "title": "Test",
-                "description": "Test tree"
+                "description": "Test tree",
+                "solution": "Test solution"
             },
             "is_default": True  # Default trees skip limit check
         }