Remove the file_manager module

[rwb27]

file_manager was a previous attempt to handle actions that created files. It is retired in favour of the Blob class. I'm not aware of any examples of file_manager being used "in the wild" and it
does not do anything that Blob can't do.

We are therefore getting rid of it as vestigial code.

[barecheck]

Barecheck - Code coverage report

Total: 89.18%

Your code coverage diff: 0.63% ▴

Uncovered files and lines

File	Lines
src/labthings_fastapi/actions/__init__.py	140, 192-198, 281-282, 358-359, 364, 372, 391-392
src/labthings_fastapi/descriptors/action.py	145-146, 150, 156, 158, 229-230, 233, 257

[rwb27]

The coverage diff was negative, so I've made some testing improvements:

1. I've tested the endpoint that lists all invocations in test_action_manager.py
2. I've excluded all the TYPE_CHECKING import blocks with an entry in .coveragerc

Neither is required to remove the file manager, but both are useful and I thought I'd be better off spending the time adding a test than justifying why I want to merge when the coverage diff is negative.

[julianstirling]

Thanks for this