Summary

• log FlowSynxException with structured logging and contextual message for the plugin config id
• return the exception message in the failure result instead of the stringified exception to match handler patterns
• aligns logging with other plugin config handlers and the guidance in Exception should be passed directly to logger instead of using ex.ToString() in UpdatePluginConfigHandler #746 to keep structured exception details intact

Testing

• dotnet test FlowSynx.sln

Closes #746.