fix: Update dapp swap comparison banner copy if rewards doesnt exist (#38317)

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## **Description**

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This PR updates dapp swap comparison banner copy if rewards does not
exists.

Note: No changelog needed as this feature not released yet.

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## **Related issues**

Fixes: MetaMask/MetaMask-planning#6349

## **Manual testing steps**

1. Trigger dapp swap comparison banner
2. See no "and earn" copy if points doesn't exist
3. See "and earn" copy if points exists

## **Screenshots/Recordings**

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### **Before**

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### **After**

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## **Pre-merge author checklist**

- [X] I've followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask
Extension Coding
Standards](https://github.com/MetaMask/metamask-extension/blob/main/.github/guidelines/CODING_GUIDELINES.md).
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## **Pre-merge reviewer checklist**

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
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- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
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<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Shows "Save with MetaMask Swaps" when rewards are absent, adding i18n
keys and updating tests accordingly.
> 
> - **UI (Confirmations)**:
> - Update `dapp-swap-comparison-banner.tsx` to render
`t('dappSwapAdvantage')` only when `rewards` exist; otherwise use
`t('dappSwapAdvantageSaveOnly')`.
> - **i18n**:
> - Add `dappSwapAdvantageSaveOnly` locale string in
`app/_locales/en/messages.json` and `app/_locales/en_GB/messages.json`.
> - **Tests**:
> - Adjust expectations in `dapp-swap-comparison-banner.test.tsx` to
check for "Save with MetaMask Swaps" without rewards and "Save and earn
with MetaMask Swaps" when rewards are provided.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
31dea0b. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: OGPoyraz

app/_locales/en/messages.json

@@ -131,7 +131,7 @@ describe('<DappSwapComparisonBanner />', () => {
     const { getByText } = render();
     expect(getByText('Market rate')).toBeInTheDocument();
     expect(getByText('MetaMask Swap')).toBeInTheDocument();
-    expect(getByText('Save and earn with MetaMask Swaps')).toBeInTheDocument();
+    expect(getByText('Save with MetaMask Swaps')).toBeInTheDocument();
     expect(getByText('Save $0.02')).toBeInTheDocument();
     expect(
       getByText('Network fees refunded on failed swaps'),
@@ -229,5 +229,6 @@ describe('<DappSwapComparisonBanner />', () => {
     });
     const { getByText } = render();
     expect(getByText(/Earn 100 points/u)).toBeInTheDocument();
+    expect(getByText('Save and earn with MetaMask Swaps')).toBeInTheDocument();
   });
 });

app/_locales/en_GB/messages.json

@@ -182,7 +182,7 @@ const DappSwapComparisonInner = () => {
             color={TextColor.TextDefault}
             variant={TextVariant.BodySm}
           >
-            {t('dappSwapAdvantage')}
+            {rewards ? t('dappSwapAdvantage') : t('dappSwapAdvantageSaveOnly')}
           </Text>
           <Text
             className="dapp-swap_text-save"