Fix interaction with Anthropic models when using it via zed.dev (#15009)

Release Notes:

- N/A

---------

Co-authored-by: Bennet <bennet@zed.dev>

crates/language_model/src/request.rs

@@ -45,7 +45,7 @@ impl LanguageModelRequest {
     pub fn preprocess(&mut self) {
         match &self.model {
             LanguageModel::OpenAi(_) => {}
-            LanguageModel::Anthropic(_) => {}
+            LanguageModel::Anthropic(_) => self.preprocess_anthropic(),
             LanguageModel::Ollama(_) => {}
             LanguageModel::Cloud(model) => match model {
                 CloudModel::Claude3Opus
@@ -54,6 +54,9 @@ impl LanguageModelRequest {
                 | CloudModel::Claude3_5Sonnet => {
                     self.preprocess_anthropic();
                 }
+                CloudModel::Custom { name, .. } if name.starts_with("anthropic/") => {
+                    self.preprocess_anthropic();
+                }
                 _ => {}
             },
         }