Start on adding support for editing via the assistant panel (#14795)

Note that this shouldn't have any visible user-facing behavior yet. The
feature is incomplete but we wanna merge early to avoid a long-running
branch.

Release Notes:

- N/A

---------

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

crates/assistant/src/completion_provider/open_ai.rs

@@ -179,7 +179,7 @@ impl LanguageModelCompletionProvider for OpenAiCompletionProvider {
         count_open_ai_tokens(request, cx.background_executor())
     }
 
-    fn complete(
+    fn stream_completion(
         &self,
         request: LanguageModelRequest,
     ) -> BoxFuture<'static, Result<BoxStream<'static, Result<String>>>> {