[Bug]: Storybook background proxy returns never-resolving promises

@georgewrmarshall is already working on this.

Since Feb 13, 2026.

## **Description**

The Storybook background proxy in `.storybook/preview.js` returns promises that never resolve or reject, causing stories that await background method calls to hang indefinitely in a loading state.

This was discovered by Cursor bot during review of PR #39539.

## **Technical Details**

**Location**: `.storybook/preview.js` lines 84-86

**Current code**:
```javascript
const proxiedBackground = new Proxy(
{},
{
get(_, method) {
return function () {
// No-op function for background calls in Storybook
return new Promise(() => {}); // ❌ This promise never resolves!
};
},
},
);
```

**Problem**:
- `new Promise(() => {})` creates a promise that never resolves or rejects
- Any story that calls `submitRequestToBackground()` and awaits the result will hang forever
- Stories cannot continue with fallback/default behavior

**Severity**: Medium
- Stories relying on background calls will be stuck in perpetual loading state
- Affects developer experience when working with stories that interact with background methods

**Proposed Fix**:
```javascript
get(_, method) {
return function () {
// No-op function for background calls in Storybook - immediately resolves
return Promise.resolve();
};
}
```

This will:
- Allow background calls to complete immediately
- Let stories continue with their fallback/default behavior
- Maintain the no-op behavior (no actual work is done)
- Prevent hanging/loading states

## **Threat Modeling Framework**

**What are we working on?**
- Fixing the Storybook background proxy to return resolved promises instead of never-resolving ones

**What can go wrong?**
- Some stories might expect the promise to never resolve (unlikely)
- Some stories might not handle resolved promises properly
- Need to verify stories still work correctly after the change

**What are we going to do about it?**
- Change `new Promise(() => {})` to `Promise.resolve()`
- Test affected stories to ensure they still render correctly
- Verify no stories are stuck in loading states

**Did we do a good job?**
- Stories load properly without hanging
- No regressions in Storybook functionality
- Developer experience is improved

## **Acceptance Criteria**

- [ ] Update `.storybook/preview.js` to return `Promise.resolve()` instead of `new Promise(() => {})`
- [ ] Run Storybook locally and verify stories load without hanging
- [ ] Run `yarn storybook` and manually test a few stories that might call background methods
- [ ] Verify CI Storybook tests pass
- [ ] No regressions in existing story functionality

## **Stakeholder Review**

- [x] Engineering (needed in most cases)
- [x] QA (automation tests are required to pass before merging PRs)

## **References**

- **Found in PR**: https://github.com/MetaMask/metamask-extension/pull/39539
- **Cursor bot comment**: https://github.com/MetaMask/metamask-extension/pull/39539#discussion_r2805182947
- **Code location**: `.storybook/preview.js:84-86`
- **Related PR**: #39539 (Storybook cleanup)