update /status response and docs

[carlad]

Closes #10130

Proposed changes:

• The ensures the response payload has the correct key/values.
• Updates the docs to conform to this.

Status (please check what you already did):

• added some tests for the functionality
• updated the documentation
• updated the changelog

[alwx]

Looks good, thanks for fixing this!

[joejuzl]

looks good!
I think we should add a changelog too though as it may affect users.

[carlad]

I've made a change to how the filename is being retrieved that differs from your suggestion @joejuzl .
The model_tar returns the full path to the file, not just the filename, so even though the tests were passing, locally the directory models was being returned in the payload.

Instead I added a model_filename attribute to the Processor, which uses the model_path to get_latest_model and then derives the os.path.basename from that.

The tests still pass, and I'm hoping that fixes things locally...

The changelog has been updated.