patch 9.1.1355: The pum_redraw() function is too complex

patch 9.1.1355: The pum_redraw() function is too complex

Problem:  The pum_redraw function is too complex and difficult to
          maintain with nested loops and mixed responsibilities handling
          both RTL and LTR text rendering.
Solution: Extracted core rendering logic into dedicated helper functions
          (pum_display_rtl_text, pum_display_ltr_text, pum_draw_scrollbar,
          pum_process_item) while preserving the original behavior. This
          improves code readability and maintainability (glepnir).

closes: #17204

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>