fix: Prevent coercing symbols to zero in the edit spending cap modal (#…

…28192)

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## **Description**

Since the `TextField` in the "edit spending cap" modal has a type
`TextFieldType.Number`, it already blocks most symbols and letters.
However, it does currently support `+`, `-` and `e` characters as they
can be used to construe numbers.

For example, when a `-` sign is introduced in the input field, the
interim value is coerced to `''`, as there is no numerical equivalent to
the sign by itself. The first part of this fix was to disable the "Save"
button on such cases. If the user wants to revoke the spending cap, they
can introduce `0`, but `''` is not a valid response.

Furthermore, when a valid number is introduced but that uses scientific
notation or `+`/`-` signs, the submission is disabled and a validation
message is shown to the user: "Enter numbers only".

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## **Related issues**

Fixes:
[#28096](#28096)

## **Manual testing steps**

1. Deploy an erc20 token contract in the test DApp
2. Trigger an approve confirmation
3. Attempt to edit the spending cap with -1, 10e10, or any others.
4. You should be prevented from submitting and see the validation
message.

## **Screenshots/Recordings**

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before and after of your change. -->

### **Before**

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### **After**

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<img width="360" alt="Screenshot 2024-10-30 at 17 46 48"
src="https://github.com/user-attachments/assets/1e05da98-e362-45ee-8fd0-a988e853677b">


## **Pre-merge author checklist**

- [ ] I've followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask
Extension Coding
Standards](https://github.com/MetaMask/metamask-extension/blob/develop/.github/guidelines/CODING_GUIDELINES.md).
- [ ] I've completed the PR template to the best of my ability
- [ ] I’ve included tests if applicable
- [ ] I’ve documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [ ] I’ve applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-extension/blob/develop/.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.

ui/pages/confirmations/components/confirm/info/approve/edit-spending-cap-modal/edit-spending-cap-modal.tsx

@@ -124,6 +124,8 @@ export const EditSpendingCapModal = ({
     decimals &&
     parseInt(decimals, 10) < countDecimalDigits(customSpendingCapInputValue);
 
+  const showSpecialCharacterError = /[-+e]/u.test(customSpendingCapInputValue);
+
   return (
     <Modal
       isOpen={isOpenEditSpendingCapModal}
@@ -171,6 +173,15 @@ export const EditSpendingCapModal = ({
               {t('editSpendingCapError', [decimals])}
             </Text>
           )}
+          {showSpecialCharacterError && (
+            <Text
+              variant={TextVariant.bodySm}
+              color={TextColor.errorDefault}
+              paddingTop={1}
+            >
+              {t('editSpendingCapSpecialCharError')}
+            </Text>
+          )}
           <Text
             variant={TextVariant.bodySm}
             color={TextColor.textAlternative}
@@ -188,7 +199,10 @@ export const EditSpendingCapModal = ({
           submitButtonProps={{
             children: t('save'),
             loading: isModalSaving,
-            disabled: showDecimalError,
+            disabled:
+              showDecimalError ||
+              showSpecialCharacterError ||
+              customSpendingCapInputValue === '',
           }}
         />
       </ModalContent>