Fix #1788: scrub exception messages from error logs to prevent PII leakage - #1791

This fixes issue mloda-ai#1788 by stripping exception strings from the traceback logged on run failure.

…ion groups

The traceback renderer recursed through __cause__/__context__ without tracking visited exceptions, so a cyclic chain raised RecursionError inside the worker except blocks and the original error was lost. Walk the chain iteratively with a seen set, render exception group members (type names and frames only), and use module-qualified names for non-builtin exception types.