MailFormatHelper::htmlToText() incorrect handling of newlines in anchor links

I tested this on Drupal 10.1.x. I added the one new test string to \Drupal\Tests\system\Functional\Mail\HtmlToTextTest::testTags and ran the test. The test did fail. So, base on that this is still valid. I also tried the proposed fix in the original patch and the test still failed.

Made a MR with the updated test and initial fix just to confirm what @quietone said. So this still needs work, but it feels fixable by someone better at regex than me!

Added this as a new test rather than modifying the existing test, and reverted the initial fix. So this is currently a test-only change.

Edited pattern and added replacing of the new lines.
\Drupal\Tests\system\Functional\Mail\HtmlToTextTest passes for me locally.
But on the build this one fails:

 1) Drupal\Tests\comment\Functional\CommentPreviewTest::testCommentPreview
    Failed asserting that '3' is an instance of interface
    "Drupal\Component\Render\MarkupInterface".

Only took a brief look and think that some comments should be added.

I have added a comment explaining the regex. Uncertain about adding a comment for the test as it's one among many and none of the others have comments (other than @todos that are.... a bit confusing anyway). Should we be adding a comment to new tests as a rule?

@quietone confirmed to add comments to new tests.

Started taking a look at this several hours ago and got distracted

But issue summary appears fine to me so removing that tag

Ran the test-only feature https://git.drupalcode.org/issue/drupal-309343/-/jobs/1501515 to show coverage

Appears feedback has been addressed and changes look fine to me.

I left a couple of comments on the MR that need addressing.