Test JSON narrowing (#4840)

### WHY are these changes introduced?

Adds test coverage for JSON type narrowing functionality to ensure proper validation of string map objects. This module was low on coverage.

### Measuring impact

How do we know this change was effective? Please choose one:

- [x] n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix
- [ ] Existing analytics will cater for this addition
- [ ] PR includes analytics changes to measure impact

### Checklist

- [x] I've considered possible cross-platform impacts (Mac, Linux, Windows)
- [x] I've considered possible [documentation](https://shopify.dev) changes

packages/cli-kit/src/public/common/ts/json-narrowing.test.ts

@@ -0,0 +1,15 @@
+import {assertStringMap} from './json-narrowing.js'
+import {BugError} from '@shopify/cli-kit/node/error'
+import {describe, expect, test} from 'vitest'
+
+describe('json-narrowing', () => {
+  test('assertStringMap', () => {
+    assertStringMap({yes: 'please'})
+  })
+  test('assertStringMap fails on null', () => {
+    expect(() => assertStringMap(null)).toThrow(BugError)
+  })
+  test('assertStringMap fails on non-object', () => {
+    expect(() => assertStringMap('hello')).toThrow(BugError)
+  })
+})