fix: safely interpolate logger context values

[michalsn]

Description
This PR fixes logger message interpolation so arbitrary PSR-3 context values do not raise PHP warnings or errors when used as placeholders.

Context values are now converted to safe strings before being passed to strtr(). Arrays and Entities are rendered with print_r() to match the logger's existing output style, Stringable values such as Time continue to use their string representation, and non-stringable objects/resources fall back to safe descriptive placeholders.

Reported in: https://forum.codeigniter.com/showthread.php?tid=94242

Ref: https://www.php-fig.org/psr/psr-3/

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

[hiSandog]

The safe interpolation behavior should be tested with the full PSR-3 variety: arrays, scalar values, Stringable objects, plain objects, resources, and null. That matters because the logger should never emit PHP warnings while handling context, but it also should not change the existing readable output style for arrays/entities.

[michalsn]

I believe we already covered a fair number of cases, and the previous behavior when it comes to the strings has not changed.