fix: preserve prototype chain in child logger info objects

[mixelburg]

Fixes #2577

The child() method uses Object.assign({}, defaultRequestMetadata, info) to merge metadata, which only copies enumerable own properties. When info is an Error subclass or any custom class instance, its prototype chain is lost — instanceof checks and custom methods no longer work.

The existing code already handles Error explicitly by copying stack, message, and cause, but this doesn't cover custom subclasses or non-Error prototypes. Using Object.create(Object.getPrototypeOf(info)) as the target for Object.assign preserves the prototype chain for all objects, not just Error instances.