#7495)

## What

The `lance-index` **test build** is currently broken on `main`:
`error[E0046]: not all trait items implemented, missing:
with_io_priority` at `rust/lance-index/src/scalar/fmindex.rs`.

`with_io_priority` was added to the `IndexStore` trait in #7449. The
`FailNewFileStore` test fixture was added later in #7422 (which branched
before #7449 landed), so it never implemented the new method. Each PR
was green on its own; together on `main` the test build fails — a
semantic merge conflict. It only surfaces in the `Build tests` step
because the fixture is `#[cfg(test)]`.

## Fix

Implement `with_io_priority` on `FailNewFileStore` by delegating to the
inner store and re-wrapping, mirroring how the fixture already delegates
its other read methods while keeping the injected `new_index_file`
failure.

## Verification

- `cargo check -p lance-index --tests` — compiles (was failing on
`main`).
- `cargo test -p lance-index --lib fmindex` — 24 passed.
- `cargo fmt -p lance-index -- --check` and `cargo clippy -p lance-index
--tests` — clean.