fix(gmail): add plain-text fallback for html send

[Lubrsy706]

Fixes #793.

Summary

• Generate a text/plain alternative whenever Gmail helpers send HTML body content.
• Keep inline-image HTML messages wrapped in multipart/related while putting a multipart/alternative body first.
• Add coverage for gmail +send --html to verify the plain fallback is present and readable.

Testing

• cargo fmt --check
• cargo test -p google-workspace-cli helpers::gmail::send::tests::test_send_html_raw_message
• cargo test -p google-workspace-cli helpers::gmail

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Latest commit: 3149b59

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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 deliverability and accessibility by ensuring that all HTML-based emails sent via the Gmail helper include a plain-text version. It introduces a lightweight utility to strip HTML tags and decode basic entities, ensuring that recipients who cannot render HTML still receive the message content.

Highlights

• Plain-text fallback: Implemented a basic HTML-to-plain-text converter to ensure emails sent as HTML include a text/plain alternative for better compatibility.
• Message structure: Updated the Gmail message builder to wrap HTML content in a multipart/alternative structure, while maintaining support for multipart/related when inline images are present.
• Testing: Added assertions to existing tests to verify that the generated email structure correctly includes the new plain-text fallback.

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[gemini-code-assist]

Code Review

This pull request introduces a plain text fallback for HTML emails sent via Gmail. It adds logic to finalize_message to generate a text/plain version of the HTML body using new helper functions html_to_plain_text and decode_basic_html_entities. This ensures better compatibility with email clients that do not support HTML. Test cases were also updated to verify the inclusion of the multipart/alternative structure and plain text content. I have no feedback to provide.