Respect --project in uv format

[jorgehermo9]

Closes #15431

This is my first contribution, so I'm sorry if I miss something! Didn't update any documentation nor added tests. Tell me if this is needed for this feature 😄

I will try to address #15430 in another PR

[zanieb]

Nice! The best way to test this is probably to copy the existing format_project and place the project in a subdirectory instead then add an unformatted file to the working directory and assert that it doesn't change but the one in the subdirectory does when you use --project.

[zanieb]

(I haven't written the uv format documentation yet, so don't worry about that part)

[jorgehermo9]

Inspired in https://github.com/astral-sh/uv/blob/35a8dd514ef22d4c71cab430c74af2c1b95fbfe3/crates/uv/tests/it/lock.rs#L25729

I'm not sure if the relative_project nomenclature applies here.

Also, there are a lot of duplicated code among these tests (mainly, the same pyproject.toml, and the formatted/unformatted code snippets). I preferred to maintain the previous style and copypaste the mock data. I'm not sure what approach do you usually follow here (to extract into constants/fixtures vs always copypaste)

[zanieb]

We could probably move them into a reusable snippet here.

[zanieb]

#15458

[jorgehermo9]

Everything addressed @zanieb! Thank you very much for your review 😃