chore: add unit test for native currency validation (#12234)

## **Description**

Adds unit test coverage for
#11693

## **Related issues**


## **Manual testing steps**


## **Screenshots/Recordings**

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before and after of your change. -->

### **Before**

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### **After**

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## **Pre-merge author checklist**

- [ ] 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).
- [ ] 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-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.

app/core/RPCMethods/wallet_addEthereumChain.test.js

@@ -291,6 +291,42 @@ describe('RPC Method - wallet_addEthereumChain', () => {
     }
   });
 
+  it('should report native currency symbol length being too long', async () => {
+    const symbol = 'aaaaaaaaaaaaaaa';
+    await expect(
+      wallet_addEthereumChain({
+        req: {
+          params: [{
+              ...correctParams,
+              nativeCurrency: { symbol, decimals: 18 },
+          }],
+        },
+        ...otherOptions,
+      }),
+    ).rejects.toThrow(
+      `Expected 1-6 character string 'nativeCurrency.symbol'. Received:\n${symbol}`,
+    );
+  });
+
+  it('should allow 1 letter native currency symbols', async () => {
+    jest.mock('./networkChecker.util');
+    jest
+      .spyOn(Engine.context.NetworkController, 'addNetwork')
+      .mockResolvedValue({ rpcEndpoints: [] });
+
+    await wallet_addEthereumChain({
+      req: {
+        params: [
+          {
+            ...correctParams,
+            nativeCurrency: { symbol: 'a', decimals: 18 },
+          },
+        ],
+      },
+      ...otherOptions,
+    });
+  });
+
   describe('Approval Flow', () => {
     it('should start and end a new approval flow if chain does not already exist', async () => {
       jest