FRE-4520: Add unit tests for notification template system

- 25 tests covering template resolution, localization fallback, variable substitution, caching, custom template registration, and edge cases
- Update package.json to use vitest for test execution
- All 25 tests passing

Co-Authored-By: Paperclip <noreply@paperclip.ing>

packages/shared-notifications/package.json

@@ -5,7 +5,8 @@
   "types": "src/index.ts",
   "scripts": {
     "build": "tsc",
-    "test": "jest",
+    "test": "vitest run",
+    "test:watch": "vitest",
     "lint": "eslint src/"
   },
   "dependencies": {
@@ -17,7 +18,8 @@
   },
   "devDependencies": {
     "@types/express": "^4.17.0",
-    "typescript": "^5.0.0"
+    "typescript": "^5.0.0",
+    "vitest": "^4.1.5"
   },
   "peerDependencies": {
     "typescript": "^5.0.0"