[BUG] Errors from marshaling agent are not returned to user due to incorrect 'diags' handling

[BSski]

Describe the bug
Diagnostics (diags) in marshalAgent func in resource_agent.go are not being correctly passed between functions, leading to errors not being returned to the user. The issue might also be present in more places - needs more investigation.

To Reproduce
Do any behavior that should result in getting an error in marshalAgent func in resource_agent.go.

Expected behavior
Errors encountered should be captured in diags and returned to the user.

System details

• All Terraform Provider versions

Additional context
Issue added for visibility, will be taken care of.