language_model_selector: Authenticate all providers up front (#25123)

This PR fixes an issue where configured language model providers would
not show up unless the configuration view was opened.

The problem was that we were filtering unauthenticated language model
providers out of the language model selector, but would only
authenticate the active provider when the selector loaded.
Authenticating the rest of the providers was deferred until the
configuration view was opened for the first time.

Closes https://github.com/zed-industries/zed/issues/21821.

Release Notes:

- Fixed an issue where configured languages models were not showing up
in the language model selector until the configuration view was opened
for the first time.