Add gpt-4o-mini, and set as default chat model

[chriswilty]

Feature Request

Description

Model gpt-4o-mini has been released, adding to the increasing unpredictability of model naming by OpenAI 🙄

Tasks:

• Add to available models, in backend/src/models/chat.ts, with context window size = 128000
• Set as default Chat model if available, in backend/src/langchain.ts function getChatModel()
• add extra tests to cover new models: refer to tests added / augmented in 912: Update available GPT models #915

Additional context

Relates to #912, which was the previous time we added models

Acceptance criteria

GIVEN I am logged in and in Sandbox level
WHEN I open the Model Configuration section in the sidebar
THEN gpt-4o-mini is selected as default chat model

GIVEN I am logged in and in Sandbox level
WHEN I open the Model Configuration section in the sidebar
AND open the Select Model dropdown
THEN I see GPT models available to me (at time of writing: gpt-3.5-turbo, gpt-4, gpt-4-turbo, gpt-4o, gpt-4o-mini)