fix(test): prevent flaky failure in test_log_langfuse_v2_handles_null_usage_values

This test has failed repeatedly in CI with:
  'Expected _add_prompt_to_generation_params to have been called once. Called 0 times.'

Root cause: _add_prompt_to_generation_params is only called when _supports_prompt()
returns True. Under cross-test state contamination in CI (parallel workers),
langfuse_sdk_version can be in an unexpected state, causing _supports_prompt() to
return False and silently skip the call (exception swallowed by the outer try/except).

Fixes:
- Use reset_mock(side_effect=True) so setUp's trace side_effect is cleared and the
  explicit return_value assignment actually takes effect
- Patch _supports_prompt on the logger instance to always return True, making the
  _add_prompt_to_generation_params assertion independent of SDK version state

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

tests/test_litellm/integrations/test_langfuse.py

@@ -268,10 +268,10 @@ class TestLangfuseUsageDetails(unittest.TestCase):
         Test that _log_langfuse_v2 correctly handles None values in the usage object
         by converting them to 0, preventing validation errors.
         """
-        # Reset the mock to ensure clean state
-        self.mock_langfuse_client.reset_mock()
-        self.mock_langfuse_trace.reset_mock()
-        self.mock_langfuse_generation.reset_mock()
+        # Reset the mock to ensure clean state; clear side_effect so return_value takes effect
+        self.mock_langfuse_client.reset_mock(side_effect=True)
+        self.mock_langfuse_trace.reset_mock(side_effect=True)
+        self.mock_langfuse_generation.reset_mock(side_effect=True)
 
         # Re-setup the trace and generation chain with clean state
         self.mock_langfuse_generation.trace_id = "test-trace-id"
@@ -283,12 +283,14 @@ class TestLangfuseUsageDetails(unittest.TestCase):
         # Ensure trace returns our mock
         self.mock_langfuse_client.trace.return_value = self.mock_langfuse_trace
         self.logger.Langfuse = self.mock_langfuse_client
-        
+
         with patch(
             "litellm.integrations.langfuse.langfuse._add_prompt_to_generation_params",
             side_effect=lambda generation_params, **kwargs: generation_params,
             create=True,
-        ) as mock_add_prompt_params:
+        ) as mock_add_prompt_params, patch.object(
+            self.logger, "_supports_prompt", return_value=True
+        ):
             # Create a mock response object with usage information containing None values
             response_obj = MagicMock()
             response_obj.usage = MagicMock()