Feat: update and add new tests for web api apps (#12714)

### What problem does this PR solve?

This PR adds missing web API tests (system, search, KB, LLM, plugin,
connector). It also addresses a contract mismatch that was causing test
failures: metadata updates did not persist new keys (update‑only
behavior).

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality)
- [x] Other (please describe): Test coverage expansion and test helper
instrumentation

test/testcases/test_web_api/conftest.py

@@ -13,6 +13,7 @@
 #  See the License for the specific language governing permissions and
 #  limitations under the License.
 #
+import os
 from time import sleep
 from ragflow_sdk import RAGFlow
 from configs import HOST_ADDRESS, VERSION
@@ -91,6 +92,15 @@ def WebApiAuth(auth):
     return RAGFlowWebApiAuth(auth)
 
 
+@pytest.fixture
+def require_env_flag():
+    def _require(flag, value="1"):
+        if os.getenv(flag) != value:
+            pytest.skip(f"Requires {flag}={value}")
+
+    return _require
+
+
 @pytest.fixture(scope="function")
 def clear_datasets(request: FixtureRequest, WebApiAuth: RAGFlowWebApiAuth):
     def cleanup():