Migrate to Rstest

Goal

Migrate Jest- or Vitest-based tests and configuration to Rstest with minimal behavior changes.

Migration principles (must follow)

1. Smallest-change-first: prefer the smallest viable change that restores test pass.
2. Config before code: prefer fixing in config/tooling/mocks before touching test logic.
3. Do not change user source behavior: avoid modifying production/business source files unless user explicitly requests it.
4. Avoid bulk test rewrites: do not refactor entire test suites when a local compatibility patch can solve it.
5. Preserve test intent: keep assertions and scenario coverage unchanged unless clearly broken by framework differences.
6. Two-phase legacy-runner lifecycle (per migrated scope): (a) While the scope being migrated is not green on Rstest, keep every Jest/Vitest dep + config/setup/workspace file untouched — they are your rollback path. (b) Once that scope is green, delete its scope-local legacy config/setup files in the same commit. Drop the shared legacy devDeps (jest, vitest, adapters, environments) only when no other scope still relies on them — in a partial / mixed-mode monorepo migration that means the final scope, not the first. Leaving files behind "for reference" creates two source-of-truth configs. Framework-specific file enumeration: see the deltas reference.
7. Literal API substitution, no shims: rewrite every vi. / jest. / vitest. call site — no global aliasing, no local rebinding, no aliased imports. Full forbidden-form list and reasoning in references/global-api-migration.md.
8. Replace on call sites, not strings: match only identifiers preceding (; after every batch edit, grep describe\(|it\(|test\( to confirm no test name string was mutated. Regex template and rationale in references/global-api-migration.md.
9. Coverage thresholds are not negotiable: never lower coverage.thresholds (lines/functions/branches/statements) to make a migrated suite pass. If thresholds fail under Rstest, investigate coverage.include / exclude / provider wiring before touching the numbers.

Workflow