fix: suppress tput stderr leak when TERM is not present

[paulbalandan]

Description
Follow-up to #10124. When stty size fails (stdin isn't a TTY), CLI::generateDimensions() falls through to tput lines / tput cols. If the spawned process inherits an environment without TERM (common in phpunit subprocess invocations and some CI runners), tput writes tput: No value for $TERM and no -T specified to stderr — exec() only captures stdout, so the message leaks. Adding 2>/dev/null to the two tput calls suppresses the noise without changing behavior: exec() still returns the empty value and the dimensions fall back to defaults.

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