fix: prevent sys.modules["langfuse"] import failures in langfuse unit tests

Three test failures caused by the real langfuse SDK import being triggered
at test time:

1. test_langfuse_prompt_management.py: Both tests create LangfusePromptManagement()
   which calls `import langfuse`. Since earlier TestLangfuseUsageDetails tests
   remove sys.modules["langfuse"] via patch.dict teardown, the real langfuse
   import runs and fails on Python 3.14 (pydantic v1 incompatibility).
   Fix: add setup_method/teardown_method to mock sys.modules["langfuse"].

2. test_langfuse.py::test_max_langfuse_clients_limit: Same root cause — creates
   LangFuseLogger() without mocking sys.modules["langfuse"].
   Fix: wrap test body with patch.dict("sys.modules", {"langfuse": mock}).

3. test_langfuse_otel.py::test_extract_langfuse_metadata_with_header_enrichment:
   Replaces sys.modules["litellm.integrations.langfuse.langfuse"] with a stub
   without restoring it, causing patch() in later tests to target the stub
   instead of the real module.
   Fix: use monkeypatch.setitem() which auto-restores after the test.

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

tests/test_litellm/integrations/langfuse/test_langfuse_prompt_management.py

@@ -1,5 +1,5 @@
 import os
-from unittest.mock import patch
+from unittest.mock import MagicMock, patch
 
 from litellm.integrations.langfuse.langfuse_prompt_management import (
     LangfusePromptManagement,
@@ -7,6 +7,20 @@ from litellm.integrations.langfuse.langfuse_prompt_management import (
 
 
 class TestLangfusePromptManagement:
+    def setup_method(self):
+        # Mock langfuse package to avoid triggering real import.
+        # The real langfuse import fails on Python 3.14 due to pydantic v1 incompatibility.
+        # This also prevents test-ordering issues when earlier tests remove sys.modules["langfuse"].
+        self._mock_langfuse = MagicMock()
+        self._mock_langfuse.version.__version__ = "3.0.0"
+        self._langfuse_patcher = patch.dict(
+            "sys.modules", {"langfuse": self._mock_langfuse}
+        )
+        self._langfuse_patcher.start()
+
+    def teardown_method(self):
+        self._langfuse_patcher.stop()
+
     def test_get_prompt_from_id(self):
         langfuse_prompt_management = LangfusePromptManagement()
         with patch.object(

tests/test_litellm/integrations/test_langfuse.py

@@ -472,8 +472,15 @@ def test_max_langfuse_clients_limit():
     """
     Test that the max langfuse clients limit is respected when initializing multiple clients
     """
+    # Mock langfuse package to avoid triggering real import.
+    # The real langfuse import fails on Python 3.14 due to pydantic v1 incompatibility,
+    # and sys.modules["langfuse"] may be absent after other tests in the suite clean up.
+    mock_langfuse = MagicMock()
+    mock_langfuse.version.__version__ = "3.0.0"
     # Set max clients to 2 for testing
-    with patch.object(langfuse_module, "MAX_LANGFUSE_INITIALIZED_CLIENTS", 2):
+    with patch.dict("sys.modules", {"langfuse": mock_langfuse}), patch.object(
+        langfuse_module, "MAX_LANGFUSE_INITIALIZED_CLIENTS", 2
+    ):
         # Reset the counter
         litellm.initialized_langfuse_clients = 0
 

tests/test_litellm/integrations/test_langfuse_otel.py

@@ -157,8 +157,12 @@ class TestLangfuseOtelIntegration:
 
         stub_module.LangFuseLogger = StubLFLogger  # type: ignore
 
-        # Register stub in sys.modules so import inside method succeeds
-        sys.modules["litellm.integrations.langfuse.langfuse"] = stub_module  # type: ignore
+        # Register stub in sys.modules so import inside method succeeds.
+        # Use monkeypatch so the real module is restored after the test runs,
+        # preventing sys.modules corruption that would break patch() targets in
+        # later tests (the patch would hit the stub while the real module's
+        # globals remain unpatched).
+        monkeypatch.setitem(sys.modules, "litellm.integrations.langfuse.langfuse", stub_module)  # type: ignore
 
         kwargs = {"litellm_params": {"metadata": {"foo": "bar"}}}
         extracted = LangfuseOtelLogger._extract_langfuse_metadata(kwargs)