Add zendesk_get_attachment tool

[patrickpingol]

Summary

• Adds a new zendesk_get_attachment tool that fetches attachment content from Zendesk
• Accepts either a full token URL (e.g. https://{subdomain}.zendesk.com/attachments/token/.../?name=file.png) or a numeric attachment ID
• When given a numeric ID, uses client.attachments.show() to resolve the content_url
• Returns images as base64 with type: "image" MCP content so clients can render them inline
• Returns all other file types as plain text with type: "text" MCP content