Response API crashes when Langfuse credentials are missing

[avirajsingh7]

After recent changes, the Response API is throwing an error when Langfuse credentials are not configured.

This issue appeared because the get_provider_credential method (updated in PR#360) was modified to raise an exception instead of returning None.
However, the existing logic in the Response API still expects it to return None, leading to unhandled exceptions.

Ensure method changed in PR360 is fixed or reverted to original version type.