Removes excess log which got through on each start of Zed
```
ERROR [agent_ui::language_model_selector] Failed to authenticate provider: Anthropic: credentials not found
```
The `AnthropicLanguageModelProvider::api_key` method returned a
`anyhow::Result` which would convert
`AuthenticateError::CredentialsNotFound` into a generic error because of
the implicit `Into` when using the `?` operator. This would then get
converted into a `AuthenticateError::Other` later.
By specifying the error type as `AuthenticateError`, we remove this
implicit conversion and the log gets removed.
Release Notes:
- N/A
tidely
d74384f6e2