mcp-sse-transport-sseclienttransport-does-not-forward-custom-headers-9569721c

THEN Modify the _startOrAuth method to merge headers from this._requestInit.headers into the fetch request for the /sse endpoint, similar to how send() does it. Ensure headers are included in the requestInit object passed to fetch.

```typescript
// In _startOrAuth, ensure requestInit includes headers:
const requestInit = {
  ...this._requestInit,
  headers: { ...this._requestInit?.headers, ...additionalHeaders },
};
```