Update optional dependency checks to reflect that jinja2 and lark are now
core dependencies of the library, not optional ones. This change:

- Simplifies dependency checking by always returning True for has_jinja2()
  and has_lark() since they are required dependencies
- Removes unnecessary import error handling for these core dependencies
- Updates type: ignore comments to only apply where needed (rich imports)
- Cleans up test suite to remove checks for jinja2/lark availability
- Fixes test mocking to align with new core dependency status

This refactor acknowledges that jinja2 (for template processing) and lark
(for devicetree parsing) are essential to the library's core functionality
and should not be treated as optional. The rich library remains optional
for enhanced display capabilities with appropriate fallback behavior.