fix: fix api fetcher balances (#6812)

## Explanation

**Current Issue:**
The `AccountsApiBalanceFetcher` had a bug caused by address format
incompatibility between controllers:
- `TokenBalancesController` uses lowercase addresses for ERC20 tokens
- `AccountTrackerController` uses checksum addresses for native tokens

The original code always checksummed all addresses, causing a wrong
token balances state

**Solution:**
Fixed by implementing conditional address formatting:
- **Native tokens**: Use checksummed addresses (compatible with
`AccountTrackerController`)
- **ERC20 tokens**: Use lowercase addresses (compatible with
`TokenBalancesController`)

Also fixed the `|| ''` fallback bug that could pass empty strings to the
`checksum()` function.

## References

- Fixes duplicate native token entries in `AccountsApiBalanceFetcher` by
ensuring controller-compatible address formatting

## Checklist

- [ ] I've updated the test suite for new or updated code as appropriate
- [x] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [x] I've communicated my changes to consumers by [updating changelogs
for packages I've
changed](https://github.com/MetaMask/core/tree/main/docs/contributing.md#updating-changelogs),
highlighting breaking changes as necessary
- [ ] I've prepared draft pull requests for clients and consumer
packages to resolve any breaking changes

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Aligns address casing between controllers by conditionally formatting
account addresses in `AccountsApiBalanceFetcher` and updating
`TokenBalancesController` state access; updates changelog.
> 
> - **Balances**:
>   - `multi-chain-accounts-service/api-balance-fetcher.ts`:
> - Conditionally format `account` per token type (checksum for native
`ZERO_ADDRESS`, original for ERC20) and propagate via `finalAccount`.
> - Track native balances using the formatted key and widen
`ProcessedBalance.account` to `ChecksumAddress | string`.
>   - `TokenBalancesController.ts`:
> - Access and write `state.tokenBalances` using `account as
ChecksumAddress` to handle mixed address formats during updates.
> - **Docs**:
> - `CHANGELOG.md`: Add "Fixed" note about address format compatibility
between `TokenBalancesController` and `AccountTrackerController` in
`AccountsApiBalanceFetcher`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
29bd9b8. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Bernardo Garces Chapero <[email protected]>

Author: salimtb

packages/assets-controllers/CHANGELOG.md

@@ -33,6 +33,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   - Reduce NFT detection API calls by 83% (from 6 calls to 1 call per 100 tokens) by eliminating collection endpoint requests
   - Remove unused collection metadata fields: `contractDeployedAt`, `creator`, and `topBid`
 
+### Fixed
+
+- Fix address format compatibility between `TokenBalancesController` and `AccountTrackerController` in `AccountsApiBalanceFetcher` ([#6812](https://github.com/MetaMask/core/pull/6812))
+
 ## [79.0.1]
 
 ### Changed

packages/assets-controllers/src/TokenBalancesController.ts

@@ -734,12 +734,14 @@ export class TokenBalancesController extends StaticIntervalPollingController<{
           const newBalance = toHex(value);
           const tokenAddress = checksum(token);
           const currentBalance =
-            d.tokenBalances[account]?.[chainId]?.[tokenAddress];
+            d.tokenBalances[account as ChecksumAddress]?.[chainId]?.[
+              tokenAddress
+            ];
 
           // Only update if the balance has actually changed
           if (currentBalance !== newBalance) {
-            ((d.tokenBalances[account] ??= {})[chainId] ??= {})[tokenAddress] =
-              newBalance;
+            ((d.tokenBalances[account as ChecksumAddress] ??= {})[chainId] ??=
+              {})[tokenAddress] = newBalance;
           }
         }
       });

packages/assets-controllers/src/multi-chain-accounts-service/api-balance-fetcher.ts

@@ -28,7 +28,7 @@ export type ChecksumAddress = Hex;
 export type ProcessedBalance = {
   success: boolean;
   value?: BN;
-  account: ChecksumAddress;
+  account: ChecksumAddress | string;
   token: ChecksumAddress;
   chainId: ChainIdHex;
 };
@@ -302,6 +302,11 @@ export class AccountsApiBalanceFetcher implements BalanceFetcher {
         }
         const account = checksum(addressPart);
         const token = checksum(b.address);
+        // Use original address for zero address tokens, checksummed for others
+        // TODO: this is a hack to get the correct account address type but needs to be fixed
+        // by mgrating tokenBalancesController to checksum addresses
+        const finalAccount: ChecksumAddress | string =
+          token === ZERO_ADDRESS ? account : addressPart;
         const chainId = toHex(b.chainId) as ChainIdHex;
 
         let value: BN | undefined;
@@ -326,14 +331,14 @@ export class AccountsApiBalanceFetcher implements BalanceFetcher {
 
         // Track native balances for later
         if (token === ZERO_ADDRESS && value !== undefined) {
-          nativeBalancesFromAPI.set(`${account}-${chainId}`, value);
+          nativeBalancesFromAPI.set(`${finalAccount}-${chainId}`, value);
         }
 
         return [
           {
             success: value !== undefined,
             value,
-            account,
+            account: finalAccount,
             token,
             chainId,
           },