Reuse workflow handlers in nested foreach

[chubes4]

Summary

• Pass the active workflow step handler map through handler context during step execution.
• Make default foreach execution reuse that active handler map for nested steps instead of rebuilding filtered defaults.
• Add smoke coverage for a constructor-injected custom handler inside a nested foreach body.

Testing

• php tests/workflow-runner-smoke.php
• composer test
• composer phpstan

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: implemented and tested nested workflow handler context propagation.