feat: pass log context as structured data to handlers instead of appending message

[michalsn]

Description
This PR passes the log context as structured data to handlers instead of appending it as a JSON string to the message inside the Logger.

HandlerInterface::handle() gains a third array $context = [] parameter carrying the full log context, with CI global context stored under HandlerInterface::GLOBAL_CONTEXT_KEY.

This allows handlers (such as ChromeHandler) to present the context as a structured object instead of a JSON-encoded string. Custom loggers can also take advantage of this to render context data natively (e.g., as expandable objects in browser dev tools or structured fields in external logging systems).

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