Dynamic width for autocomplete suggestions dropdown widget

[hexycat]

Summary

The width of the AutocompleteDropdown widget is now explicitly set on every call of the set_items method, determined by the longest item in the visible suggestion list plus border/padding/scrollbar offsets.

Reason

Currently the width of the dropdown is always set to min-width regardless of its content or the max-width value. The likely reason is related to the width: auto CSS property of the dropdown combined with the width CSS property of the parent container, which results in the width being constrained to min-width.

This is especially noticeable when table names share long identical prefixes.
In such case suggestions may look like this:

After changes:

Changes

1. Add _update_width method to AutocompleteDropdown class.
   It computes and sets the width of the dropdown from the longest visible suggestion plus border/padding/scrollbar offsets, clamped between MIN_WIDTH (25) and MAX_WIDTH (80).
   The method is called at the end of set_items, so the width is updated whenever the list of visible suggestions changes
2. Add UI tests tests/ui/test_autocomplete_dropdown.py covering the new expected width behaviour of AutocompleteDropdown