mesh-1629: fix auth pseudocode and add request body / response schema…

[matt-mercer]

…s for send/get message

Summary

• Routine Change
• ❗ Breaking Change
• 🤖 Operational or Infrastructure Change
• ✨ New Feature
• ⚠️ Potential issues that might be caused by this change

Add any other relevant notes or explanations here. Remove this line if you have nothing to add.

Reviews Required

• Dev
• Test
• Tech Author
• Product Owner

Review Checklist

ℹ️ This section is to be filled in by the reviewer.

• I have reviewed the changes in this PR and they fill all or part of the acceptance criteria of the ticket, and the code is in a mergeable state.
• If there were infrastructure, operational, or build changes, I have made sure there is sufficient evidence that the changes will work.
• I have ensured the changelog has been updated by the submitter, if necessary.

[github-actions]

This branch is work on a ticket in the NHS Digital AMB JIRA Project. Here's a handy link to the ticket:

MESH-1629