Fix Langfuse logger test mock setup (#17588)

* Fix test_log_langfuse_v2_handles_null_usage_values test failure

The test was failing because the logger's Langfuse client wasn't properly
mocked. Even though sys.modules was mocked, the logger's __init__ method
creates its own Langfuse client instance that wasn't using the test's mock.

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

This ensures the test can properly verify that _log_langfuse_v2 correctly
converts None usage values to 0 by allowing the mock's generation method
to be called and asserted.

Fixes: AssertionError: Expected 'generation' to have been called once. Called 0 times.

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(