Slack MCP plugin authentication failure

Description

The Slack MCP plugin (plugin:slack:slack) fails to authenticate. After going through the /mcp → select plugin:slack:slack → Authenticate flow, the plugin remains disconnected and Slack MCP tools are not available.

Steps to reproduce

1. Run /mcp in Claude Code
2. Select plugin:slack:slack
3. Choose "Authenticate"
4. Auth flow does not complete successfully — plugin stays disconnected

Expected behavior

OAuth flow completes and Slack MCP tools (message posting, channel reading, etc.) become available for use.

Environment

• OS: macOS (Darwin 25.3.0)
• CLI: Claude Code
• Model: Claude Opus 4.6 (1M context)