Default OAuth scope to mcp for dynamic client registration

What does this MR do and why?

Default OAuth scope to mcp for dynamic client registration

When dynamic OAuth applications request authorization without a scope parameter, default to mcp scope per RFC 7591. This simplifies MCP server configuration.

Additional info

I've also tried to test with this configuration, but didn't yet succeed. It seems to me we need to apply some additional changes to make it work. This can probably be done separately.

{
  "mcpServers": {
    "GitLab-GDK-http": {
      "type": "http",
      "url": "https://gdk.test:3443/api/v4/mcp"
    }
  }
}

References

Related to #570482 (closed)

How to set up and validate locally

Use simplified mcpServers configuration:

{
  "mcpServers": {
    "GitLab-GDK": {
      "command": "npx",
      "args": [
        "-y",
        "mcp-remote",
        "https://gdk.test:3443/api/v4/mcp"
      ],
      "env": {
        "NODE_TLS_REJECT_UNAUTHORIZED": "0"
      }
    }
  }
}

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