[ml service] Refactor: Decompose ml_single_open_custom function for better maintainability

This commit refactors the monolithic ml_single_open_custom function (200+ lines)
into smaller, focused helper functions to improve code readability and
maintainability. The main function is now structured as a clear sequence
of steps, each handled by a dedicated helper function.

Signed-off-by: hyunil park <hyunil46.park@samsung.com>

Author: songgot