docs(tests): update test documentation to reflect current test structure and commands

Update README.md in the test directory, including:
- More detailed test type descriptions
- Updated test commands
- Changed frontend testing tool from pytest to Vitest
- Added frontend test directory layout explanation

tests/README.md

@@ -3,30 +3,30 @@
 The test tree is organized with a clear backend/frontend split:
 
 - `tests/backend`
-  - backend unit tests
-  - backend integration tests
+  - backend unit tests, integration tests, and contract tests
+  - driven by `pytest`
   - regression tests directly related to `py-src/data_formulator/**`
 - `tests/frontend`
-  - frontend-related contract tests driven by `pytest`
-  - focused on request/response boundary behavior
-  - not intended for React component-level unit tests yet
+  - frontend unit tests powered by **Vitest** + **@testing-library/react** (jsdom)
+  - covers pure functions, Redux selectors, and React component rendering
+  - see `tests/frontend/README.md` for directory layout details
 
 ## Suggested Commands
 
-Run all tests:
-
-```bash
-pytest
-```
-
-Run backend tests only:
+Run backend tests (pytest):
 
 ```bash
 pytest tests/backend
 ```
 
-Run frontend contract tests only:
+Run frontend tests (Vitest):
 
 ```bash
-pytest tests/frontend
+npm test
+```
+
+Run frontend tests in watch mode:
+
+```bash
+npm run test:watch
 ```