fix(Model Manager): refuse to download a file when there's insufficient space

[keturn]

A partial fix for Invoke exhausting disk space, this PR to the DownloadQueueService causes it to refuse to download if the advertised file size is bigger than the available space.

I tried to put this at a higher level, in ModelInstallService._install_remote_model, but despite its attempt to calculate the total_bytes of the install job at that time, in practice it doesn't have that information.

I say partial fix because the model installer copies the files from the temporary directory to the model directory, causing you to temporarily need twice as much disk space—that can be a separate issue.

It is also short-sighted in that if given an install job involving multiple files, it may spend the time to fully download earlier files before realizing it cannot complete all of them.

QA Instructions

• install a model from URL or HF
• if you want to be thorough, try to install a model that's bigger than your remaining free disk space. Or you can temporarily hack the code's free_space calculation to simulate that condition.

Merge Plan

N/A

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

[psychedelicious]

Thanks!