PR Summary

Improves error extraction used by model fallback decisions to prevent misleading stringification of non-string errors.

• Refactors getErrorMessage to handle Error, strings, primitives, symbols, and object.message, otherwise return empty string (no implicit String(err))
• Updates CHANGELOG.md with the agents note for this fix

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Landed via merge.\n\n- Gate: pnpm lint && pnpm build && pnpm test\n- Commits: 6481806, 9ecef09\n- Merge commit: 0a4cb0d\n\nThanks @steipete!