fix: Correct extension field type handling per review feedback
- Changed parseExtensions to use Record<string, unknown> instead of any
- Added type assertions at call sites for type safety
- Restored SpecificationExtensions to CallbackObject, PathsObject, ResponsesObject
- Kept PathsObject with [path: /${string}] template literal type
- Extended index signatures with | unknown to satisfy TypeScript
Co-authored-by: mrlubos <12529395+mrlubos@users.noreply.github.com>