fix(perps): use existing position leverage as fallback for initial leverage (MetaMask#23619)

## **Description**

This PR fixes an issue where the `PerpsOrderProvider` would not properly
initialize leverage when modifying an existing position. Previously, if
`initialLeverage` was not explicitly provided via navigation params, the
form would not fall back to the existing position's leverage value,
which could lead to incorrect leverage initialization when modifying
positions.


## **Changelog**

CHANGELOG entry: Fixed leverage initialization when modifying existing
perpetual positions

## **Related issues**

Fixes:

Jira Task: https://consensyssoftware.atlassian.net/browse/TAT-2201

## **Manual testing steps**

```gherkin
Feature: Perps order form leverage initialization

  Scenario: user modifies existing position with leverage
    Given user has an existing perpetual position with 5x leverage
    And user navigates to modify the position
    Then the order form should display 5x leverage as the initial value even when the internet connection is lost
```

## **Screenshots/Recordings**

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

### **Before**

No visible change

### **After**

No visible change

## **Pre-merge author checklist**

- [x] 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).
- [x] I've completed the PR template to the best of my ability
- [x] I’ve included tests if applicable
- [x] I’ve documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [x] 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]
> Fallback to `existingPosition.leverage.value` when `initialLeverage`
is undefined in `PerpsOrderProvider` initialization.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
49f8fee. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: michalconsensys

app/components/UI/Perps/contexts/PerpsOrderContext.tsx

@@ -34,7 +34,7 @@ export const PerpsOrderProvider = ({
     initialAsset,
     initialDirection,
     initialAmount,
-    initialLeverage,
+    initialLeverage: initialLeverage ?? existingPosition?.leverage?.value,
     initialType,
   });