Restore nextest in agent test instructions (#25745)
## Summary
Following the discussion in
https://github.com/astral-sh/ruff/pull/25734, we previously removed
`cargo nextest` from `AGENTS.md` because it was not available in local
agent sandboxes and the fallback attempts were error-prone. I checked
the current local Codex sandbox, and it seems to work for me without
issue?
This restores `cargo nextest` as the preferred test runner. I've added
`cargo test` fallback commands for environments where nextest is
unavailable.
