fix: avoid undefined STDERR and STDIN outside the CLI

[paulbalandan]

Description

Fixes #5006

PHP defines STDIN, STDOUT and STDERR only in the CLI SAPI, and CLI::init() provides a stand-in for STDOUT alone. So CLI::error() ends a web request with Undefined constant "CodeIgniter\CLI\STDERR" when a command runs through command() in a controller. Outside the CLI it now writes through STDOUT, so command() returns the message like any other output. CLI::clearScreen() on Windows and SignalTrait::getProcessState() had the same problem, and both are guarded the same way.

The fix checks is_cli() rather than defining the missing constants, because defined('STDIN') is a common way to detect the CLI and defining it in web requests would break those checks.

Only CLI::error() has a regression test. Under PHPUnit STDIN is always defined, and the clearScreen() path is Windows-only.

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