Migrate ESLint to flat config format (v9) and update CI workflow

.github/workflows/ci.yml

@@ -25,7 +25,7 @@ jobs:
         run: npm ci
 
       - name: Run ESLint
-        run: npx eslint "src/**/*.ts" --ext .ts
+        run: npx eslint "src/**/*.ts"
 
       - name: Run Prettier check
         run: npx prettier --check "src/**/*.ts" "test/**/*.ts"