Prefix code mode output with success or failure message and include error stack

[pakrym-oai]

Summary

• send node runner metadata (max output tokens, stack) back to Rust so we can prepend status headers and ship script errors cleanly
• add truncation helpers plus JS config for per-execution budgets and enforce them before the final response
• update tooling docs/specs and tests to cover the new headers, truncation budget, and failure formatting

Testing

• Not run (not requested)