Fix regression that caused Anthropic custom models to error (#15329)

/cc: @bennetbo 

Release Notes:

- N/A

Co-authored-by: Nathan <nathan@zed.dev>

crates/language_model/src/provider/anthropic.rs

@@ -174,7 +174,7 @@ impl AnthropicModel {
         }
 
         Request {
-            model: self.model.clone(),
+            model: self.model.id().to_string(),
             messages: request
                 .messages
                 .iter()