Display error and prevent submit if a user does not provide all required API keys in chat panel setup

[JasonWeill]

Problem

On initial setup of the chat UI, one or more text boxes appear, so that users can provide their credentials.

These text boxes are not required inputs, and I can submit the form without providing the required key, and I see no errors when I do so. Jupyter AI's chat panel remains nonfunctional.

Proposed Solution

Indicate that the user must provide a value for all authentication credential inputs. Consider adding an informative message about what is required; this can also be a link to the docs.

Block submission of the form, or reject form submissions with an error, if all required creds are not provided.