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

/cc: @bennetbo 

Release Notes:

- N/A

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

crates/anthropic/src/anthropic.rs

@@ -100,21 +100,13 @@ impl From<Role> for String {
 
 #[derive(Debug, Serialize)]
 pub struct Request {
-    #[serde(serialize_with = "serialize_request_model")]
-    pub model: Model,
+    pub model: String,
     pub messages: Vec<RequestMessage>,
     pub stream: bool,
     pub system: String,
     pub max_tokens: u32,
 }
 
-fn serialize_request_model<S>(model: &Model, serializer: S) -> Result<S::Ok, S::Error>
-where
-    S: serde::Serializer,
-{
-    serializer.serialize_str(&model.id())
-}
-
 #[derive(Serialize, Deserialize, Debug, Eq, PartialEq)]
 pub struct RequestMessage {
     pub role: Role,