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/conftest.py

@@ -14,6 +14,83 @@
 #  limitations under the License.
 #
 
+import importlib
+import sys
+import types
+
+
+def _make_stub_getattr(module_name):
+    def __getattr__(attr_name):
+        message = f"{module_name}.{attr_name} is stubbed in tests"
+
+        class _Stub:
+            def __init__(self, *_args, **_kwargs):
+                raise RuntimeError(message)
+
+            def __call__(self, *_args, **_kwargs):
+                raise RuntimeError(message)
+
+            def __getattr__(self, _name):
+                raise RuntimeError(message)
+
+        setattr(sys.modules[module_name], attr_name, _Stub)
+        return _Stub
+
+    return __getattr__
+
+
+def _install_rag_llm_stubs():
+    rag_llm = sys.modules.get("rag.llm")
+    if rag_llm is not None and getattr(rag_llm, "_rag_llm_stubbed", False):
+        return
+
+    try:
+        rag_pkg = importlib.import_module("rag")
+    except Exception:
+        rag_pkg = types.ModuleType("rag")
+        rag_pkg.__path__ = []
+        rag_pkg.__package__ = "rag"
+        rag_pkg.__file__ = __file__
+        sys.modules["rag"] = rag_pkg
+
+    llm_pkg = types.ModuleType("rag.llm")
+    llm_pkg.__path__ = []
+    llm_pkg.__package__ = "rag.llm"
+    llm_pkg.__file__ = __file__
+    sys.modules["rag.llm"] = llm_pkg
+    rag_pkg.llm = llm_pkg
+
+    llm_pkg.__getattr__ = _make_stub_getattr("rag.llm")
+
+    for submodule in ("cv_model", "chat_model"):
+        full_name = f"rag.llm.{submodule}"
+        sub_mod = sys.modules.get(full_name)
+        if sub_mod is None or not isinstance(sub_mod, types.ModuleType):
+            sub_mod = types.ModuleType(full_name)
+            sys.modules[full_name] = sub_mod
+        sub_mod.__package__ = "rag.llm"
+        sub_mod.__file__ = __file__
+        sub_mod.__getattr__ = _make_stub_getattr(full_name)
+        setattr(llm_pkg, submodule, sub_mod)
+
+    llm_pkg._rag_llm_stubbed = True
+
+
+def _install_scholarly_stub():
+    if "scholarly" in sys.modules:
+        return
+    stub = types.ModuleType("scholarly")
+
+    def _stub(*_args, **_kwargs):
+        raise RuntimeError("scholarly is stubbed in tests")
+
+    stub.scholarly = _stub
+    sys.modules["scholarly"] = stub
+
+
+_install_rag_llm_stubs()
+_install_scholarly_stub()
+
 import pytest
 import requests
 from configs import EMAIL, HOST_ADDRESS, PASSWORD, VERSION, ZHIPU_AI_API_KEY