NIAD-3273 Fix DuplicatePrimaryKeyError on MHS Outbound retry by using upsert instead of insert

[chiaramapellimt]

What

Replace insert_one with find_one_and_replace using upsert=True in MongoPersistenceAdaptor.add. Updated the corresponding test to reflect the new behaviour.

Relates ticket: https://nhse-dsic.atlassian.net/browse/NIAD-3273

Why

When an MHS Outbound request failed with a 500, the record was already written to MongoDB. Retrying with the same Message ID would then fail with a DuplicatePrimaryKeyError because insert_one does not allow duplicate keys. Using upsert means the retry will overwrite the existing record instead of erroring, allowing the request to be resent successfully.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Internal change (non-breaking change with no effect on the functionality affecting end users)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the Changelog with details of my change in the UNRELEASED section if this change will affect end users