Fix: Extract text from HTML part for message preview

[Siam11651]

Contribution Summary

Linked Issue/Ticket: Fixes: #5890

Description

Now email preview will prioritize HTML over plaintext. Be it notification or any folder.

Screen Shots

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• This contribution does not include any changes created or assisted by AI.
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Contribution Checklist

• I have read and affirm that my contribution adheres to Mozilla’s Community Participation Guidelines
• This contribution is in Kotlin where possible
• This contribution does not use merge commits
• This contribution adheres to the existing codestyle.
• This contribution does not break existing unit tests.
• This contribution includes tests for any new functionality, and maintains tests for any updated functionality.
• This contribution adheres to our Engineering process (RFC/Technical Design/ADR)
• This PR has a descriptive title and body that accurately outlines all changes made, and contains a reference to any issues that it fixes (e.g Fixes Extract text from HTML part for message preview #5890).

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