tests: improve RAGFlow coverage based on Codecov report (#13200)

### What problem does this PR solve?

Codecov’s coverage report shows that several RAGFlow code paths are
currently untested or under-tested. This makes it easier for regressions
to slip in during refactors and feature work.
This PR adds targeted automated tests to cover the files and branches
highlighted by Codecov, improving confidence in core behavior while
keeping runtime functionality unchanged.

### Type of change

- [x] Other (please describe): Test coverage improvement (adds/extends
unit and integration tests to address Codecov-reported gaps)

test/testcases/test_web_api/test_plugin_app/test_llm_tools.py

@@ -13,6 +13,11 @@
 #  See the License for the specific language governing permissions and
 #  limitations under the License.
 #
+import importlib.util
+import sys
+from pathlib import Path
+from types import ModuleType
+
 import pytest
 from common import plugin_llm_tools
 from configs import INVALID_API_TOKEN
@@ -40,3 +45,54 @@ class TestPluginTools:
         res = plugin_llm_tools(WebApiAuth)
         assert res["code"] == 0, res
         assert isinstance(res["data"], list), res
+
+
+class _DummyManager:
+    def route(self, *_args, **_kwargs):
+        def decorator(func):
+            return func
+        return decorator
+
+
+def _load_plugin_app(monkeypatch):
+    repo_root = Path(__file__).resolve().parents[4]
+    common_pkg = ModuleType("common")
+    common_pkg.__path__ = [str(repo_root / "common")]
+    monkeypatch.setitem(sys.modules, "common", common_pkg)
+
+    stub_apps = ModuleType("api.apps")
+    stub_apps.login_required = lambda func: func
+    monkeypatch.setitem(sys.modules, "api.apps", stub_apps)
+
+    stub_plugin = ModuleType("agent.plugin")
+
+    class _StubGlobalPluginManager:
+        @staticmethod
+        def get_llm_tools():
+            return []
+
+    stub_plugin.GlobalPluginManager = _StubGlobalPluginManager
+    monkeypatch.setitem(sys.modules, "agent.plugin", stub_plugin)
+
+    module_path = Path(__file__).resolve().parents[4] / "api" / "apps" / "plugin_app.py"
+    spec = importlib.util.spec_from_file_location("test_plugin_app_unit", module_path)
+    module = importlib.util.module_from_spec(spec)
+    module.manager = _DummyManager()
+    spec.loader.exec_module(module)
+    return module
+
+
+@pytest.mark.p2
+def test_llm_tools_metadata_shape_unit(monkeypatch):
+    module = _load_plugin_app(monkeypatch)
+
+    class _DummyTool:
+        def get_metadata(self):
+            return {"name": "dummy", "description": "test"}
+
+    monkeypatch.setattr(module.GlobalPluginManager, "get_llm_tools", staticmethod(lambda: [_DummyTool()]))
+    res = module.llm_tools()
+    assert res["code"] == 0
+    assert isinstance(res["data"], list)
+    assert res["data"][0]["name"] == "dummy"
+    assert res["data"][0]["description"] == "test"