BQML backend and .env file variables (explore-assistant-extension)

[rikaando]

When using BQML as the backend in explore-assistant-extension, leaving the VERTEX_AI_ENDPOINT and VERTEX_CF_AUTH_TOKEN variables in the .env file with their default values (ex. ) causes an error. It appears the code uses the presence of values in these variables to determine whether the backend is Cloud Functions or BQML.

To resolve this, I suggest one of the following solutions:

1. Introduce a backend selection environment variable in the extension (frontend). This would allow users to explicitly specify the backend they are using, avoiding any ambiguity based on the .env file.

2. Update the README with instructions to comment out or leave blank the VERTEX_AI_ENDPOINT and VERTEX_CF_AUTH_TOKEN variables when not using Cloud Functions. This would provide clearer guidance to users on how to configure the extension for BQML.

Implementing either of these solutions would improve the user experience and prevent confusion when setting up the explore-assistant-extension with BQML.