Fix Claude Sonnet 4 model ID (#31505)

This PR is a follow-up to
https://github.com/zed-industries/zed/pull/31415 that fixes the model ID
for Claude Sonnet 4.

With the release of the Claude 4 models, the model version now appears
at the end.

Release Notes:

- N/A

crates/assistant_settings/src/assistant_settings.rs

@@ -1018,7 +1018,7 @@ mod tests {
                 AssistantSettings::get_global(cx).default_model,
                 LanguageModelSelection {
                     provider: "zed.dev".into(),
-                    model: "claude-4-sonnet".into(),
+                    model: "claude-sonnet-4".into(),
                 }
             );
         });