chore: expose `files_by_partition` to public api

[edmondop]

Description

This documentation describes the usage of files_by_partition but that API doesn't seem to be surfaced from the underlying rust implementation

• [ ] Add unit tests once confirmed this makes sense

[github-actions[bot]]

ACTION NEEDED

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[ion-elgreco]

@edmondop can you fix the linting issues? Then we can merge it

[edmondop]

I fixed the linting, but I think I should add a test since I have grown the API surface before this get merged.

I submitted to verify if I was wrong in reading the documentation or if a method was missing from the public api

[edmondop]

@ion-elgreco looks also there is a flaky concurrency test

[ion-elgreco]

@edmondop let me know when you added the test