Add MCP tools to download files from email links

Add functionality to extract and download files from URLs found in email
bodies (in addition to existing embedded MIME attachments).

New tools:
- extract_download_links: Parses email body to find URLs, classifies them
  by file extension and download keywords
- download_link_attachment: Downloads a file from a URL with redirect
  handling and filename detection from headers
- download_all_link_attachments: Batch downloads all likely download links
  from an email

Features:
- Detects API-based download links (e.g., /api/export, ?format=csv)
- Extracts filename from Content-Disposition header or URL
- Maps content-type to file extension for proper naming
- Handles redirects and timeout configuration

This enables handling emails that include download links rather than
direct attachments (e.g., "click here to download your report").

Author: claude