fix: correctly parse flow slug for invite-to-pay cases for GOV Pay metadata

[jessicamcinchak]

Fixes failed regression tests on main this morning: https://github.com/theopensystemslab/planx-new/actions/runs/7191369572

Now passing on this branch: https://github.com/theopensystemslab/planx-new/actions/runs/7193346466

Gov Pay was throwing this error:

code: "P0102"
description: "Invalid attribute value: metadata. Values must be no greater than 100 characters long"
field: "metadata"

This was only effecting invite-to-pay cases, not "regular" payments initiated on the frontend - as these always have direct access to the flowSlug in state and do not rely on parsing the returnUrl.

Metadata is now correctly captured:

[github-actions]

Removed vultr server and associated DNS entries