Migrate tests to Vitest (#109)

## Summary
- replace Jest with Vitest 4.1.0 for the TypeScript test suite
- simplify and restructure the tests with clearer describe blocks and
shared mock state
- remove the old Jest config and use a minimal Vitest config

## Verification
- npm run build
- npm run test

Author: j178

CONTRIBUTING.md

@@ -40,7 +40,7 @@ Run the unit tests:
 npm test
 ```
 
-This runs the TypeScript tests directly with Jest via `ts-jest`.
+This runs the TypeScript tests directly with Vitest.
 
 ## Validate