Fix addable for InputfieldPageAutocomplete in single-page mode

The Bug:
When InputfieldPageAutocomplete is configured for single-page selection (derefAsPage > 0), it operates in a no_list mode. In this mode, if the user hits Enter to create a new page using the "allow new pages to be created from field" (addable) feature, the JS incorrectly tries to dynamically build the selector for the hidden _add_items input.

It does this by taking the text input's ID (which has an _input suffix, e.g., Inputfield_publisher_input) and stripping only "Inputfield_" from it (id.replace('Inputfield_', '')).
This results in an invalid base name (e.g., publisher_input) instead of the true field name (publisher). Consequently, the JavaScript searches for the wrong ID (#_publisher_input_add_items instead of #_publisher_add_items), silently fails to find the input, and the new page is never submitted or created on save.

The Fix:
This PR extracts the exact base realName directly from the hidden data input's name attribute dynamically in init():

var realName = $value.attr('name') ? $value.attr('name').replace('[]', '') : id.replace('Inputfield_', '');

By using realName across the script, the JS reliably locates the actual _add_items field (#_publisher_add_items), completely fixing the addable page creation for single-page autocomplete fields.