fix(runner): preserve multiline message text in results truncation

[kemilsoderberg]

Summary

Changes results, show and diff to show multiple lines of messages up to the message-char limit, fixes #625

Type of change

• Bug fix
• New feature
• Refactor / internal change
• Documentation
• Build / CI / tooling
• Breaking change

Checklist

• I have read the Contribution Guide and the AI and Automated Contribution Policy.
• The change is focused on a single concern (no unrelated refactors or formatting noise).
• Tests for the change have been added or updated, and hatch run test:test passes locally (RF matrix against the default Python) — or N/A for documentation-only / non-code changes. See Running Tests for faster iteration options and the full matrix.
• hatch run lint:all passes (also enforced by the pre-commit hooks if installed) — or N/A for documentation-only / non-code changes.
• Documentation has been updated where relevant (user docs, code comments, README).
• Generated files (if any) were regenerated with the documented script, not edited by hand.
• Commits follow Conventional Commits and are cryptographically signed (git commit -S, GPG/SSH).

AI / tooling disclosure

• No AI/automation was used beyond ordinary editor autocomplete.
• AI/automation was used; tool(s):
  • I reviewed the generated output manually, understand it, and verified it locally.