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

/cc: @bennetbo 

Release Notes:

- N/A

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

crates/completion/src/completion.rs

@@ -27,7 +27,7 @@ pub struct LanguageModelCompletionProvider {
 const MAX_CONCURRENT_COMPLETION_REQUESTS: usize = 4;
 
 pub struct LanguageModelCompletionResponse {
-    pub inner: BoxStream<'static, Result<String>>,
+    inner: BoxStream<'static, Result<String>>,
     _lock: SemaphoreGuardArc,
 }