loadSupergraphSdlFromStorage throws TS2869

### Issue Description

**Node version:** v22.14.0 (lts/jod)
**pnpm version**: 10.6.4

While building the application, I encounter the following error:

> TS2869: Right operand of ?? is unreachable because the left operand is never nullish.

The error is throw in the following file in gateway-js:
`supergraphManagers/UplinkSupergraphManager/loadSupergraphSdlFromStorage.ts`

At line 163:
```typescript
throw new UplinkFetcherError(fetchErrorMsg + result.status + ' ' + e.message ?? e);
```

## Possible solution

Change the line to as follows (notice the parenthesis):
```typescript
throw new UplinkFetcherError(fetchErrorMsg + result.status + ' ' + (e.message ?? e));
```

or as follows:
```typescript
throw new UplinkFetcherError(`${fetchErrorMsg} ${result.status} ${e.message ?? e}`);
```

### Link to Reproduction

https://github.com/qumberrizvi/escrow-backend

### Reproduction Steps

Install dependencies:
```bash
pnpm install
```

Start microservices:
```bash
pnpm start:dev:ms

Start in supergraphManagers/UplinkSupergraphManager/loadSupergraphSdlFromStorage.ts at line 163 and inspect the TS2869 diagnostic during the application build. Reproduce with the reported pnpm install and build or development startup steps; done means the unreachable-nullish-operand error is resolved while the resulting UplinkFetcherError still includes the relevant message.

Written by the indexing model from the issue text.