fix: MUSD-456 prevent mUSD stale approval cleanup from rejecting active confirmations (MetaMask#27155)

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## **Description**
The `useMusdConversionStaleApprovalCleanup` hook was rejecting pending
mUSD approvals whenever the app returned from background — including
when the user simply tapped an in-app "terms apply" link that opened the
browser. This caused the confirmation screen to enter a stuck loading
state.

This PR adds a screen-aware guard with deeplink detection so the hook
can distinguish between:

- Returning from an in-app link (browser round-trip) → preserves the
approval
- Incoming deeplink that navigates the user away → rejects the approval
- Not on a confirmation screen → rejects immediately (existing behavior)

Detection uses React Native's Linking url event listener, which fires
for incoming deeplinks but not for returns from `Linking.openURL()`.

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## **Changelog**

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CHANGELOG entry: fixed mUSD conversion confirmation no longer gets stuck
when tapping external links that briefly background the app.

## **Related issues**

Fixes: [MUSD-456: Opening "terms apply" link on Max and Custom convert
rejects the transaction and navigates
back](https://consensyssoftware.atlassian.net/browse/MUSD-456)

## **Manual testing steps**

```gherkin
Feature: mUSD conversion stale approval cleanup

  Scenario: user taps external link on confirmation screen and returns
    Given user is on the mUSD conversion confirmation screen with a pending approval

    When user taps the "terms apply" link and returns from the browser
    Then the pending approval is preserved and the confirmation screen remains functional

  Scenario: user returns to app via deeplink while on confirmation screen
    Given user is on the mUSD conversion confirmation screen with a pending approval

    When the app is reopened via an incoming deeplink
    Then the stale pending approval is rejected and the user is not stuck in a disabled state

  Scenario: user returns from background when not on confirmation screen
    Given user has a pending mUSD conversion approval and is not on a confirmation screen

    When the app returns from background
    Then the stale pending approval is rejected immediately
```

## **Screenshots/Recordings**

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before and after of your change. -->

### **Before**

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https://github.com/user-attachments/assets/3898d1c2-54d4-420d-ae11-9ebc47a80ef6

### **After**

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https://github.com/user-attachments/assets/7197fac6-ef08-4089-b02b-7316fe922992

## **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]
> **Medium Risk**
> Changes the conditions under which pending mUSD approvals are
auto-rejected and adds deeplink detection based on `Linking` events,
which could affect approval lifecycle/navigation if misclassified. Scope
is limited and covered by expanded unit tests.
> 
> **Overview**
> Prevents `useMusdConversionStaleApprovalCleanup` from auto-rejecting
pending mUSD approvals when the user is *actively in the confirmation
flow* (e.g. `REDESIGNED_CONFIRMATIONS` / `TOOLTIP_MODAL`) and the app
returns from background without an incoming deeplink.
> 
> Adds `Linking` `url` event tracking to distinguish true deeplink
resumes (reject approvals and optionally `goBack` from the orphaned
confirmation screen) from browser round-trips, refactors rejection into
a helper, and significantly expands tests to cover listener lifecycle,
screen-aware guarding, deeplink flag reset behavior, and post-rejection
navigation.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
032186e. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: Matt561