Add test handlers, option to choose mock language model for testing

[andrii-i]

Problem

Currently to test something involving interaction with language model or to test chat UI, real API key or locally hosted language model is required. This is not optimal for testing.

Proposed Solution

Add test handlers, option to choose mock language model for testing.

[JasonWeill]

A testing language model, aka a mock testing model, would need to return predictable output for any prompt.