fix: suppress stty stderr leak in CLI::generateDimensions() when stdin is not a TTY

[paulbalandan]

Description
When stdin isn't a TTY (PHPUnit, CI, piped output), stty writes Inappropriate ioctl for device to stderr — PHP's exec() only captures stdout, so that message leaked to the console. Redirecting stderr fixes it without affecting the fallback logic, since exec() still returns empty stdout and falls through to tput.

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