Fix urls for affordances in the server thing descriptions

[julianstirling]

Fixing the bug where the server thing_descriptions route returns the wrong url for affordances. The / wass missing between the thing name and the action/property name. (See #221).

This is currently 2 commits. The first is a failing unit test, the second is the fix.

[barecheck]

Barecheck - Code coverage report

Total: 95.07%

Your code coverage diff: 0.04% ▴

Uncovered files and lines

File	Lines
src/labthings_fastapi/server/__init__.py	167, 317

[rwb27]

Thanks for spotting, fixing, and testing this. I'd love to eliminate code that manually manipulated URLs as per #211, but this is a good fix for the immediate problem.