Add handle_exception method to log errors during GenerationJob execution

[TheRealNeil]

Add missing handle_exception class method to Rescue concern

Fixes #306

Summary

• Add handle_exception class method to ActiveAgent::Concerns::Rescue so that GenerationJob#handle_exception_with_agent_class has something to call

Root cause

GenerationJob#handle_exception_with_agent_class calls klass.handle_exception(exception) (a class method), but the Rescue concern only defines instance methods. This causes a NoMethodError when a job-level exception is raised during generate_later.

Fix

Add a handle_exception class method to the Rescue concern's class_methods block. It logs the exception class, message, and first 10 lines of the backtrace via Rails.logger.error.

Test plan

• Trigger an unhandled exception during generate_later and confirm it is logged rather than raising NoMethodError
• Confirm rescue_from instance-level handlers still work as expected