…or Ondo swaps when coming from Rewards (#30419)

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## **Description**

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This change pre-fills the correct source token when attempting to swap
into Ondo assets from the Rewards campaign.

## **Changelog**

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CHANGELOG entry: made it easier to get started swapping Ondo GM assets
from Rewards

## **Related issues**

Fixes: n/a

## **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**

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### **After**

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## **Pre-merge author checklist**

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- [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
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if applicable
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guidelines](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.

#### Performance checks (if applicable)

- [ ] I've tested on Android
  - Ideally on a mid-range device; emulator is acceptable
- [ ] I've tested with a power user scenario
- Use these [power-user
SRPs](https://consensyssoftware.atlassian.net/wiki/spaces/TL1/pages/edit-v2/401401446401?draftShareId=9d77e1e1-4bdc-4be1-9ebb-ccd916988d93)
to import wallets with many accounts and tokens
- [ ] I've instrumented key operations with Sentry traces for production
performance metrics
- See [`trace()`](/app/util/trace.ts) for usage and
[`addToken`](/app/components/Views/AddAsset/components/AddCustomToken/AddCustomToken.tsx#L274)
for an example

For performance guidelines and tooling, see the [Performance
Guide](https://consensyssoftware.atlassian.net/wiki/spaces/TL1/pages/400085549067/Performance+Guide+for+Engineers).

## **Pre-merge reviewer checklist**

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<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Changes the token preselection logic passed into `goToSwaps` for
Rewards-driven Ondo swaps, which can impact swap routing and user
balances if chain/token mapping is wrong. Scope is contained to the Ondo
RWA selector and covered by updated unit tests.
> 
> **Overview**
> Improves Rewards → Ondo *open position* flow by choosing an
appropriate prefilled swap source token based on the selected
destination asset’s chain: **prefer USDY on Ethereum when the user holds
a balance**, otherwise fall back to a per-chain default (`USDC` on
`eip155:1`, `USDT` on `eip155:56`).
> 
> Updates the selector and after-hours confirmation paths to pass this
computed source into `goToSwaps`, and revises tests/mocks to validate
chain-aware defaults and remove the previous “undefined source”
expectations.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
b1019e7. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->