feat: add toJSON serialization to RuntimeError (Issue #155)

[rafaio1]

Summary

Adds toJSON() method to RuntimeError so that code, details, and stack survive JSON.stringify for lossless logging and API serialization.

Changes

• ✅ Added toJSON() returning { name, code, message, details, stack }
• ✅ All fields preserved through JSON round-trip
• ✅ Stack trace included in serialized output

Tests Added

• ✅ code and details present in JSON.stringify output
• ✅ Stack trace included in toJSON() result
• ✅ Undefined details handled correctly
• ✅ Full round-trip preservation verified

Verification

All 4 tests pass locally via npx vitest run tests/errors/runtime-error-tojson.test.ts.

Closes #155