feat: add nextest, rstest and insta test tooling

[rozsazoltan]

Summary

Adds a modern Rust test stack to the toolchain using cargo-nextest, rstest, and insta.

Changes

• Use cargo nextest run --workspace as the main unit/integration test runner.
• Keep doctests covered through a separate cargo test --workspace --doc step.
• Install nextest in the release workflow before running Rust tests.
• Add cargo:cargo-nextest to mise.toml for local quality checks.
• Update hk.pkl and repository-managed hk generation to use nextest.
• Add rstest and insta as repository test dependencies.
• Convert datarose schema diagnostics tests to parameterized rstest cases.
• Add insta snapshots for schema diagnostics and generated hk Rust test steps.
• Update repository workflow rendering tests for the mise-backed quality gate.
• Update README quality instructions to document the new Rust test flow.

Notes

cargo-nextest runs the normal Rust #[test] unit and integration tests with better runner UX and CI behavior. Doctests remain on Cargo because nextest does not run doctests.