chore: retain src input amount after switching tokens (#29709)

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## **Description**

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## **Related issues**

Fixes:   https://consensyssoftware.atlassian.net/browse/MMS-1798

## **Manual testing steps**

1. Load bridge page
2. Select amount, src token and dest token
3. Click token switch button to swap src and dest selections
4. Verify that src amount is preserved

## **Screenshots/Recordings**

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before and after of your change. -->

### **Before**

Amount is cleared 

### **After**
Amount is preserved

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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
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ui/pages/bridge/prepare/prepare-bridge-page.tsx

@@ -489,7 +489,6 @@ const PrepareBridgePage = () => {
                   : undefined;
               toChainClientId && dispatch(setActiveNetwork(toChainClientId));
               dispatch(setFromToken(toToken));
-              dispatch(setFromTokenInputValue(null));
               fromChain?.chainId && dispatch(setToChain(fromChain.chainId));
               fromChain?.chainId && dispatch(setToChainId(fromChain.chainId));
               dispatch(setToToken(fromToken));