feat: Add HTML body fallback when text/plain is not available

[peteWT]

Summary

• Adds HTML body fallback when text/plain is not available in emails
• Many email clients (Outlook, Gmail, etc.) send HTML-only emails without a text/plain alternative, causing get_emails_content to return empty body fields
• Strips HTML tags to convert to readable text

Changes

• Collects text/html content as a fallback when parsing multipart emails
• Adds _strip_html() helper function to convert HTML to readable text
• Handles single-part HTML emails the same way

Test plan

• Tested locally with emails from Gmail and Outlook that previously returned empty bodies
• Verified plain text emails still work correctly (text/plain is preferred)
• Unit tests (would appreciate guidance on test structure)

Fixes #113

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🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 19.23077% with 21 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
mcp_email_server/emails/classic.py	19.2%	20 Missing and 1 partial ⚠️

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