Add .seek_read_exact(), .seek_write_all() to std::os::windows::fs::FileExt

[json420]

Tracking issue: #162868

This adds .seek_read_exact() to std::os::windows::fs::FileExt, an exact port of read_exact_at() for std::os::unix::fs::FileExt.

And it adds .seek_write_all() to std::os::windows::fs::FileExt, an exact port of write_all_at() for std::os::unix::fs::FileExt.

Finally, it adds six tests.

First there is a smoke test for seek_read_exact(), seek_write_all(), an exact port of the test for the .seek_read(), .seek_write() methods. This is to make sure we are getting the same pass-through behavior through the lens of established tests.

Then there are five tests for the windows::fs::FileExt trait itself, covering all seek_read_exact(), seek_write_all() corner cases except io::ErrorKind::Interrupted (advise welcome on that one). These tests use mock implementations of the traits seek_read() and seek_write() methods.

No LLMs were used in writing this PR.

[rustbot]

Thanks for the pull request, and welcome! The Rust Project has assigned @clarfonthey (or someone else) to review your changes, you should hear from them (or someone else) within the next two weeks.

Please see the contribution instructions and our LLM policy for more information.

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: @ChrisDenton, libs
• @ChrisDenton, libs expanded to 13 candidates
• Random selection from 6 candidates

[clarfonthey]

LGTM minus test splitting.

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[json420]

@rustbot ready

[clarfonthey]

@bors r+ rollup

Thank you!

[rust-bors]

📌 Commit 6ac2b72 has been approved by clarfonthey

It is now in the queue for this repository.

[json420]

@clarfonthey No, thank you! Contributing to Rust is, like, a wildly pleasant experience for a first timer! Seriously. Thanks!