chore: Add a lock check to CI

[xanderbailey]

Which issue does this PR close?

We don't currently check in CI that the lock file is up-to-date and that seems wrong to me. Checked that without updating the lock file (currently out of date) that this fails https://github.com/apache/iceberg-rust/actions/runs/26219087517/job/77149052535

This fails with:

note: pass `--verbose` to see 142 unchanged dependencies behind latest
error: the lock file /Users/runner/work/iceberg-rust/iceberg-rust/Cargo.lock needs to be updated but --locked was passed to prevent this
If you want to try to generate the lock file without accessing the network, remove the --locked flag and use --offline instead.
Error: Process completed with exit code 101.

• Closes #.

What changes are included in this PR?

Are these changes tested?

[CTTY]

LGTM!