chore: Default Perps local and production builds to mainnet (#21650)

## **Description**

Quality of life change to default local environments to run Perps on
mainnet. Devs can still go into Developer options and toggle to testnet
if they wish.

## **Changelog**

CHANGELOG entry: default perps controller to mainnet

## **Related issues**

Fixes:

## **Manual testing steps**

```gherkin
Feature: my feature name

  Scenario: user [verb for user action]
    Given [describe expected initial app state]

    When user [verb for user action]
    Then [describe expected outcome]
```

## **Screenshots/Recordings**

<!-- If applicable, add screenshots and/or recordings to visualize the
before and after of your change. -->

### **Before**

<!-- [screenshots/recordings] -->

### **After**

<!-- [screenshots/recordings] -->

## **Pre-merge author checklist**

- [ ] I’ve followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask Mobile
Coding
Standards](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/CODING_GUIDELINES.md).
- [ ] I've completed the PR template to the best of my ability
- [ ] I’ve included tests if applicable
- [ ] I’ve documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [ ] I’ve applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.

## **Pre-merge reviewer checklist**

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Sets `isTestnet` default to `false` (mainnet) in `PerpsController` and
updates tests to reflect the new default.
> 
> - **PerpsController**
(`app/components/UI/Perps/controllers/PerpsController.ts`):
> - Change default `isTestnet` to `false` in
`getDefaultPerpsControllerState`.
> - **Tests**
(`app/components/UI/Perps/controllers/PerpsController.test.ts`):
>   - Update constructor test to assert `isTestnet` defaults to `false`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
d419c14. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: gambinish

app/components/UI/Perps/controllers/PerpsController.test.ts

@@ -158,6 +158,7 @@ describe('PerpsController', () => {
       expect(controller.state.accountState).toBeNull();
       expect(controller.state.connectionStatus).toBe('disconnected');
       expect(controller.state.isEligible).toBe(false);
+      expect(controller.state.isTestnet).toBe(false); // Default to mainnet
     });
 
     it('should read current RemoteFeatureFlagController state during construction', () => {

app/components/UI/Perps/controllers/PerpsController.ts

@@ -254,7 +254,7 @@ export type PerpsControllerState = {
  */
 export const getDefaultPerpsControllerState = (): PerpsControllerState => ({
   activeProvider: 'hyperliquid',
-  isTestnet: __DEV__, // Default to testnet in dev
+  isTestnet: false, // Default to mainnet
   connectionStatus: 'disconnected',
   accountState: null,
   positions: [],