MCP OAuth callback displays a blank page

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### Dify version

1.15.0

### Cloud or Self Hosted

Cloud

### Steps to reproduce

1. In the console, add an MCP server `https://mcp.notion.com/mcp` that uses OAuth authorization.
2. Start the authorization flow and approve access at the OAuth provider.
3. Let the provider redirect the browser to `/console/api/mcp/oauth/callback` with valid `code` and `state` query parameters.
4. Observe the page shown after the callback completes.

The API callback stores the OAuth data and redirects to `${CONSOLE_WEB_URL}/oauth-callback`. The frontend route renders an empty `