[paulbalandan]

Description
Follow-up to #10669. forStderr() cloned the deferral flag, so a write through the STDERR copy cleared only the copy's flag. The original's next write to STDOUT then wrote a second blank line. This shows after a prompt followed by BaseCommand::showError(), which writes through the copy via CLI::redirectToStderr().

• The copy now shares the flag with the original, so the blank line is written once by whichever writes first.
• OutputTest assertions on multi-line plain text now compare the joined output as a nowdoc, with the stream order checked separately, so blank lines are visible at a glance.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide