fix(gmail): send html bodies as multipart alternative

[Lubrsy706]

Fixes #793.

Summary

• emit a text/plain fallback alongside text/html for gmail +send --html
• keep inline CID images inside multipart/related while nesting multipart/alternative as the root related part
• preserve existing attachment behavior for mixed messages

Tests

• cargo fmt --check
• cargo test -p google-workspace-cli finalize_message
• cargo test -p google-workspace-cli test_attachment_with_html_body

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Latest commit: 439de46

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request improves email compatibility by ensuring that HTML-formatted emails sent via the Gmail integration include a text/plain alternative. This change helps email clients that do not support HTML or prefer plain text to render the message correctly, while maintaining proper support for inline CID images and existing attachment handling.

Highlights

• Multipart/Alternative Support: Implemented multipart/alternative support for HTML emails, ensuring a text/plain fallback is provided alongside the text/html content.
• HTML to Plain Text Conversion: Added a new utility function html_to_plain_text to strip HTML tags and generate a clean plain-text version of the email body.
• Message Structure Refinement: Updated finalize_message to correctly nest multipart/alternative within multipart/related for inline images, while preserving existing attachment behavior.

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[gemini-code-assist]

Code Review

This pull request introduces a mechanism to automatically generate a plain text version of HTML email bodies using a new html_to_plain_text helper function. It updates the finalize_message function to include both text/plain and text/html parts within a multipart/alternative structure when sending HTML emails, ensuring better compatibility with email clients. Corresponding tests were added and updated to verify the correct generation of these multipart messages. I have no feedback to provide.