Fix case-insensitive charset detection for ISO-2022-JP

[neconohitomi]

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI

Summary

Fixes ISO-2022-JP email conversion when the charset is provided in lowercase.

Problem

Some emails declare their charset as ISO-2022-JP, but Horde can return the charset as lowercase (iso-2022-jp).

Converter previously checked the charset against mb_list_encodings() using a case-sensitive comparison. As a result, iso-2022-jp was not recognized as an mbstring encoding and the conversion fell back to iconv().

On the affected Alpine/libiconv environment, iconv() fails to convert these ISO-2022-JP messages, resulting in garbled email content.

Fix

Make the charset comparison case-insensitive when checking the available mbstring encodings.

The existing charset normalization and conversion logic are otherwise unchanged.

Tests

Added a regression test covering lowercase iso-2022-jp.

Tested with:

• ConverterTest: 15 tests, 30 assertions — OK
• Full Mail unit test suite: 1518 tests, 4378 assertions — OK

Fixes #13472

[kesselb]

Please move array_map(strtoupper) into the mbEncodings function.

[kesselb]

strtoupper($charset) should be done in normalizedCharset. At best by altering the map to also use uppercase and flip lowerCharset to upperCharset and return that version as fallback.

[kesselb]

Good finding, Thanks for your pr 👍

[kesselb]

The commit message needs a DCO signoff and the Assisted-By trailer.

It's often enough to point your Agent https://github.com/nextcloud/mail/blob/main/AGENTS.md#commit-message-format and https://github.com/nextcloud/mail/pull/13641/checks?check_run_id=101341532634 to resolve it.

[neconohitomi]

The commit message needs a DCO signoff and the Assisted-By trailer.

It's often enough to point your Agent https://github.com/nextcloud/mail/blob/main/AGENTS.md#commit-message-format and https://github.com/nextcloud/mail/pull/13641/checks?check_run_id=101341532634 to resolve it.

Thanks for the review and for pointing out that the current commit is missing the required DCO Signed-off-by and Assisted-by trailers.

I used an AI agent (ChatGPT) while preparing this PR, and this makes me wonder about the following situation. I would appreciate your advice.

According to AGENTS.md, review changes should be submitted as fixup commits without force-pushing. However, the DCO check reports that the existing commit itself is missing the sign-off.

What would be the preferred way to fix the DCO/Assisted-by trailers in this PR while following the review workflow?

I can prepare the required fixup commit if that is the preferred approach.