fix: split oversized final results into indexed chunks

[LIU9293]

Summary

• add a shared splitResultMessage helper that splits final output at 3000 chars, prefers newline/space boundaries, and prefixes multipart chunks as (i/N)
• update runtime final publishing to send multipart chunks as separate thread messages instead of a single truncated message
• remove underscore-wrapped fallback notices and add focused tests for chunking and index prefix behavior

Testing

• bun test packages/core/test/result-message.test.ts packages/core/test/request-runner.test.ts