Add structured error details extraction

[Fryuni]

Summary

Add a public helper for extracting structured, serializable details from unknown errors, including nested causes. Document its API and cover fallback and recursive behavior with focused tests.

Changes

• Add ErrorDetails with message, optional stack, and recursive cause fields
• Add extractErrorDetails with configurable fallback handling
• Document the helper and its return type with JSDoc
• Test string, unsupported, empty-message, stack, and nested-cause cases

Testing

• npm test -- --runInBand test/utilities.test.ts — 26 tests passed
• npm run validate
• npm run lint

[github-actions]

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