Reapply Langfuse logger test mock setup fix (#17591)

Reapplies the fix from commit a885e21543 that was
reverted in 6c9556be67.

The original revert was done because the test was flaky and giving false
negatives. This fix properly mocks the Langfuse client to ensure the test
can correctly verify that _log_langfuse_v2 converts None usage values to 0.

Changes:
- Add mock_langfuse_client.client attribute to prevent errors during init
- Add trace_id to mock_langfuse_generation for proper return value handling
- Remove redundant mock setup code
- Explicitly set logger.Langfuse to mock client after initialization
- Set logger.langfuse_sdk_version to ensure _supports_* methods work correctly

tests/test_litellm/integrations/test_langfuse.py

@@ -35,8 +35,11 @@ class TestLangfuseUsageDetails(unittest.TestCase):
 
         # Create mock objects
         self.mock_langfuse_client = MagicMock()
+        # Mock the client attribute to prevent errors during logger initialization
+        self.mock_langfuse_client.client = MagicMock()
         self.mock_langfuse_trace = MagicMock()
         self.mock_langfuse_generation = MagicMock()
+        self.mock_langfuse_generation.trace_id = "test-trace-id"
 
         # Setup the trace and generation chain
         self.mock_langfuse_trace.generation.return_value = self.mock_langfuse_generation
@@ -63,22 +66,13 @@ class TestLangfuseUsageDetails(unittest.TestCase):
         sys.modules["langfuse"] = self.mock_langfuse
         sys.modules["langfuse"].Langfuse = self.mock_langfuse_class
 
-        # Mock the Langfuse client
-        self.mock_langfuse_client = MagicMock()
-        self.mock_langfuse_trace = MagicMock()
-        self.mock_langfuse_generation = MagicMock()
-
-        # Setup the trace and generation chain
-        self.mock_langfuse_trace.generation.return_value = self.mock_langfuse_generation
-        self.mock_langfuse_client.trace.return_value = self.mock_langfuse_trace
-
-        # Mock the Langfuse class
-        self.mock_langfuse_class = MagicMock()
-        self.mock_langfuse_class.return_value = self.mock_langfuse_client
-        self.mock_langfuse.Langfuse = self.mock_langfuse_class
-
         # Create the logger
         self.logger = LangFuseLogger()
+        
+        # Explicitly set the Langfuse client to our mock
+        self.logger.Langfuse = self.mock_langfuse_client
+        # Ensure langfuse_sdk_version is set correctly for _supports_* methods
+        self.logger.langfuse_sdk_version = "3.0.0"
 
         # Add the log_event_on_langfuse method to the instance
         def log_event_on_langfuse(