language_model: Return AuthenticateErrors from LanguageModelProvider::authenticate (#25126)

This PR updates the `LanguageModelProvider::authenticate` method to
return an `AuthenticateError` instead of an `anyhow::Error`.

This allows us to model the "credentials not found" state explicitly as
`AuthenticateError::CredentialsNotFound`, which enables the caller to
check for this state and act accordingly.

Planning to use this in #25123 to silence errors about missing
credentials when authenticating providers in the background.

Release Notes:

- N/A

Cargo.lock

@@ -6951,6 +6951,7 @@ dependencies = [
  "serde_json",
  "smol",
  "strum",
+ "thiserror 1.0.69",
  "ui",
  "util",
 ]