[FIX] mail: chatwindow search autocomplete with long name overflowing

When we search for a name in the chat window having a very long name
of the user will overflowing the screen. The autocomplete dropdown list is
overflowing and rendering out of the screen.

With this commit, we have resized the width of autocomplete dropdown list with
the width of the outer element's width as it's standard behavior.

Task : 2449115

closes odoo/odoo#66238

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>