/auto (#16696)

Add `/auto` behind a feature flag that's disabled for now, even for
staff.

We've decided on a different design for context inference, but there are
parts of /auto that will be useful for that, so we want them in the code
base even if they're unused for now.

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/language_model/src/provider/anthropic.rs

@@ -319,7 +319,7 @@ impl AnthropicModel {
         };
 
         async move {
-            let api_key = api_key.ok_or_else(|| anyhow!("missing api key"))?;
+            let api_key = api_key.ok_or_else(|| anyhow!("Missing Anthropic API Key"))?;
             let request = anthropic::stream_completion(
                 http_client.as_ref(),
                 &api_url,