Handle workflow step handler exceptions

[chubes4]

Summary

• Catch Throwable during workflow step execution and normalize it into a failed step/run error with error_type: handler_exception.
• Preserve the runner recorder lifecycle so throwing handlers produce a terminal failed update instead of escaping.
• Add workflow runner smoke coverage for throwing handlers and terminal recorder updates.

Testing

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

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: implemented and tested workflow exception terminal-record handling.