fix: satisfy PHPStan 2.2.3

[memleakd]

Description

This PR fixes PHPStan failures after phpstan/phpstan started resolving to 2.2.3.

PHPStan now narrows ob_get_contents() / ob_get_clean() / ob_get_flush() / ob_get_length() to non-false while output buffering is active: phpstan/phpstan-src#5909

Because of that, a few existing output-buffering tests now report redundant ob_get_level() > 0 checks, and helper return types can be narrowed from false|string to string.

There is no behavior change.

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

[michalsn]

While these changes make sense for the test files, I don't think command() should be treated the same way. It executes arbitrary command code that may alter the output buffer state, so we cannot assume the buffer remains active.

[memleakd]

While these changes make sense for the test files, I don't think command() should be treated the same way. It executes arbitrary command code that may alter the output buffer state, so we cannot assume the buffer remains active.

Good point, thanks. Updated it to handle that case explicitly.