[FSSDK-11649] Fix FSC failed tests for CMAB

[Mat001]

The issue in the CMAB error handling path within the GetDecision method. When the CMAB API returns an error, the previous implementation attempted to append reasons from the failed API call to the accumulated reasons list. This caused FSC tests to fail because:

• The reasons weren't properly propagated to downstream consumers
• When API errors occurred, the Decision object contained incorrect or missing information
• FSC tests expected specific error reasons to be present in the Decision object
  Fix
  Modified the error handling path to return a clean Decision object with properly initialized reasons

https://jira.sso.episerver.net/browse/FSSDK-11649