Add an optional fetch param to SSEClientTransportOptions to allow custom fetch implementation.

Motivation and Context

For SSE transport, previously the eventSourceInit param allowed to customize the fetch function used by EventSource. However, the send method still use the native fetch, which was not customizable. In some use cases, we might want both to use a custom fetch implementation. For example, in Tauri/Capacitor applications , the frontend runs in a webview, and the native fetch cannot perform cross-origin requests directly. Therefore, a custom fetch function is needed to bypass CORS.

How Has This Been Tested?

npm test passed. Additionally, I have used the modified transport in AIaW with a custom fetch function to test and verify cross-origin functionality.

Breaking Changes

No

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context