chore: some client.aget calls should use use_http_response

What does this merge request do and why?

Updates the following files to use client.aget with use_http_response=True instead of the legacy response format

duo_workflow_service/tools/job.py
duo_workflow_service/tools/merge_request.py
duo_workflow_service/tools/pipeline.py
duo_workflow_service/tools/previous_context.py

How to set up and validate locally

Should behave the same as this is a simple refactor

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.
• If this change requires executor implementation: verified that issues/MRs exist for both Go executor and Node executor or confirmed that changes are backward-compatible and don't break existing executor functionality.

Related to #1507