Título: [adobe-for-creativity] OAuth authentication fails with HTTP 403

## Description

When attempting to authenticate the `adobe-for-creativity` plugin in Claude Code,
the OAuth flow fails with HTTP 403.

## Steps to reproduce

1. Install plugin: `/plugin adobe-for-creativity`
2. Run `/reload-plugins`
3. Attempt authentication via Claude Code SDK

## Error received

From authenticate tool:
SDK auth failed: HTTP 403: Invalid OAuth error response:
SyntaxError: JSON Parse error: Unexpected EOF. Raw body: empty.

From MCP server directly:
```json
{
"jsonrpc": "2.0",
"error": {
"code": 401,
"message": "Authorization header is required"
}
}

Environment

- Claude Code version: latest
- OS: Windows 11
- Plugin version: 1.1.0
- MCP server URL: https://adobe-creativity.adobe.io/mcp

Expected behavior

OAuth flow completes and tools become available.

Possible cause

The Claude Code SDK OAuth client may not have permission to initiate
the OAuth flow with Adobe's server, or the server may be in restricted access.

Start with the adobe-for-creativity plugin's authenticate tool and its Claude Code SDK OAuth flow, reproducing the failure against https://adobe-creativity.adobe.io/mcp. Compare the SDK response with the MCP server's authorization error and verify that authentication completes and the tools become available.

Written by the indexing model from the issue text.