fix(autocomplete): use selected item template in single mode

[xonaib]

Summary

• use the selectedItem template text for the single-mode AutoComplete input display
• keep the existing optionLabel fallback when no selectedItem template is present
• add a regression test for selecting an item in single mode with pTemplate="selecteditem"

Fixes #938

Test plan

• pnpm --filter @openng/optimus-ui test:unit --include=src/autocomplete/autocomplete.spec.ts --reporters=progress
• pnpm run build:lib