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/conftest.py

@@ -174,13 +174,15 @@ async def test_tree(client, auth_headers):
                     "id": "solution1",
                     "type": "solution",
                     "title": "Test Confirmed",
-                    "description": "This is a test tree"
+                    "description": "This is a test tree",
+                    "solution": "Test confirmed - this is a test tree"
                 },
                 {
                     "id": "solution2",
                     "type": "solution",
                     "title": "Not a Test",
-                    "description": "This should not happen"
+                    "description": "This should not happen",
+                    "solution": "Not a test - this should not happen"
                 }
             ]
         }