Fix value selector widget for IN and NOT_IN operator

[pmeier]

Reported in #499 (comment)

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1. The vertical spacing should allow for at least three values to be visible at all times.

   

2. The value extracted from the widget is wrong. For example if I select the following filter in the modal

   

   I get the following output in the chat history

   

   There are two issues here:

   1. It did pick a different document as I did select. In my test I couldn't make out a pattern other than it only ever picked a single one.
   2. The value is still a string rather than a list of strings. This leads to errors in the backend, e.g. Chroma.