uplift(release): fix: utf8 decoding (#11510)

[wmontwe]

Contribution Summary

Linked Issue/Ticket: #11508
Original pull request: #11510

Description

Uplifts the UTF-8 MIME header decoding fix to release. Raw headers are explicitly decoded as UTF-8 instead of relying on Mime4j's ISO-8859-1 toString() behavior. This fixes incorrectly displayed and saved non-ASCII attachment filenames.

Includes the original regression tests for all affected parsing paths.

Testing

• ./gradlew :mail:common:test :legacy:core:test :mail:common:spotlessCheck :legacy:core:spotlessCheck --no-daemon

Risk

Low. The change is narrowly scoped to raw MIME header decoding and includes regression tests. No localizable strings or database migrations are changed.

AI Disclosure

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

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 updated functionality
• This contribution adheres to our Engineering process
• This PR has a descriptive title and body

[wmontwe]

[Approval Request]
Original Issue/Pull request: #11508 / #11510
Regression caused by (issue #): #11005
User impact if declined: Non-ASCII attachment filenames continue to be displayed and saved incorrectly on release.
Testing completed (on daily, beta, etc.): Original fix landed on main; targeted unit tests and Spotless checks pass locally on this release uplift branch. The beta uplift is #11539.
Introduces or modifies localizable strings (yes/no): No
Skips a database migration (yes/no): No
Risk to taking this patch (and alternatives if risky): Low. The change is narrowly scoped to raw MIME header decoding and includes regression tests. The alternative is to let the fix ride the release train.