chore: upgrade TypeScript from 4.8.2 to 5.9.3

[a9a4k]

Summary

Upgrades the TypeScript toolchain from v4.8.2 to v5.9.3 (latest 5.x release) along with all related TypeScript ecosystem dependencies.

Changes

• typescript: 4.8.2 → 5.9.3
• @typescript-eslint/parser: 6.7.0 → 7.18.0
• @typescript-eslint/eslint-plugin: 6.7.0 → 7.18.0
• @types/node: 18.11.18 → 20.17.10
• ts-node: 10.5.0 → 10.9.2
• eslint-plugin-unused-imports: 3.0.0 → 4.3.0

Motivation

This upgrade is necessary to support Zod v4, which requires TypeScript 5.0+ for const type parameters. It also brings performance improvements and better type inference from TypeScript 5.x.

Testing

✅ All validations passed:

• Type checking: No errors
• Build: 0 errors, dual format (CJS + ESM) working correctly
• Tests: 248/248 passed in 20 test suites
• Linting: Clean
• Runtime: Both CJS require() and ESM import() working
• Declaration files: Verified correct generation (.d.ts and .d.mts)
• Critical postprocess-files.cjs: Working correctly with upgraded parser

Risk Assessment

LOW RISK - No breaking changes:

• No enums, decorators, or experimental features in codebase
• All existing TypeScript patterns are fully compatible with TS5
• Complex shim architecture and dual package setup work correctly
• No changes required to tsconfig.json

Additional Notes

• The eslint-plugin-unused-imports was upgraded from v3 to v4 to ensure compatibility with @typescript-eslint v7
• The build system's postprocess-files.cjs (which uses @typescript-eslint/parser for AST manipulation) works correctly with the upgraded parser

[sdserranog]

@a9a4k everything looks good! Thanks for doing this.