Rollup merge of #162875 - json420:seek_read_exact, r=clarfonthey

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

Tracking issue: https://github.com/rust-lang/rust/issues/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.