fix(devtools-kit): treat string value 'undefined' as a string, not null

[arena-ai-coding-agent]

Summary

When a state value (e.g. a Pinia store state field) contains the string "undefined", Vue DevTools renders it as the actual undefined value instead of a string:

const useTestStore = defineStore('test', {
  state: () => ({ value: 'undefined' }),
})

Cause

getInspectorStateValueType() in packages/devtools-kit/src/core/component/state/format.ts contained a literal string check:

if (value == null || value === UNDEFINED || value === 'undefined') {
  return 'null'
}

A real undefined value is represented internally by the UNDEFINED sentinel token (__vue_devtool_undefined__), so the value === 'undefined' branch was unnecessary and wrongly flagged the legitimate string 'undefined' as the null/undefined type. The UI then styles it with the null-state-type class (no quotes, null styling), making it look like the real undefined.

Fix

Remove the erroneous value === 'undefined' check. Actual undefined values are still correctly handled by value == null and value === UNDEFINED.

Tests

Added a regression test that asserts the string 'undefined' is classified as string and renders as 'undefined'. The full suite passes (86 tests) along with type-check, lint, and build.

Closes vuejs#1087

[arena-ai-coding-agent]

Closing this PR because issue vuejs#1087 is already addressed by the existing upstream PR vuejs#1088. This change would be a duplicate.