CI: Run slow/unstable tests nightly

[anakin87]

Our CI is getting slow and sometimes unstable.

This is mostly related to integrations tests that:

• call unstable external services (e.g., Hugging Face)
• perform model inference using CPU

A possible approach

While in the long run we could think about moving some components to core integrations, we can put a mitigation strategy in place.

• Mark slow/unstable tests with a specific marker
• Create a workflow to run these tests, nightly and on PRs that touch specific files
• Include this workflow in branch protection rules to block merging if it fails (related: create a skipper workflow)
• bonus: if we carefully select tests, we might avoid running Tika and installing system dependencies for Whisper during ordinary integration tests; these 2 steps make integration tests considerably slower

[anakin87]

This work has been primarily done in #9267, #9296, and #9300.
I also took the opportunity to improve some slow tests (#9297) and review all our integration tests (#9306), converting some into unit tests and removing others.

Some data about CI jobs run times (comparing a PR before the refactoring with one after)

Job	Before	After
Unit / ubuntu-latest	2m 20s	2m 6s
Unit / macos-latest	2m 4s	1m 41s
Unit / windows-latest	5m 42s	5m 2s
Integration / ubuntu-latest	4m 7s	2m 7s
Integration / macos-latest	4m 46s	2m 12s
Integration / windows-latest	5m 39s	6m 11s

Overall, execution times have improved.
Windows integrations tests have recently become slightly slower, for unrelated factors, mainly cache recovery time (that we should keep monitored as these tests are now the main bottleneck).

Slow/unstable integration tests only run on PRs that modify related files and nightly.

Job	Duration
Slow Tests / ubuntu-latest	4m 55s
Slow Tests / macos-latest	4m 11s
Slow Tests / windows-latest	5m 55s

I hope the main outcome will be a more stable CI, as unstable tests have been isolated and are now executed only under specific conditions.