[Minor] Update libucl with automatic stack management

[Minor] Update libucl with automatic stack management

Merge changes from upstream libucl commit 26bec99:
- Add UCL_STACK_AUTOMATIC flag for automatic stack preservation
- Modify stack cleanup to conditionally preserve automatic stacks
- Enable parsing of included files without outer braces

This allows parsing of UCL configurations that include files with
content like 'key = value;' without requiring explicit braces,
improving compatibility with various UCL file formats.