feat: add per-call context logging with Throwable normalization

[michalsn]

Description
This PR adds opt-in per-call context logging controlled by three new Config\Logger options: $logContext, $logContextTrace, and $logContextUsedKeys.

When enabled, context keys not used as message placeholders are passed to handlers as structured data. Throwable instances are automatically normalized to a meaningful array instead of serializing to an empty object.

All options default to false, preserving existing behavior.

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]

Thank you @paulbalandan