fix: prevent AttributeError in joblib worker config sync

[Ayush10]

Summary

Closes #2038

When backtests spawn joblib worker processes, register_from_C can crash with:

AttributeError: No such registered in self._config

This happens because the worker's fresh Config instance may not have the _registered key available when C.registered is accessed.

Fix

Replace the direct attribute access with a safe getattr call:

# Before
if C.registered and skip_register:

# After
if getattr(C, "registered", False) and skip_register:

This defaults to False when the attribute is missing, allowing the worker to proceed with config synchronization from the main process.

Test Plan

• Single-line defensive fix
• Behavior unchanged when registered attribute exists
• Gracefully handles missing attribute by defaulting to False