open_ai: Trim newline before "data:" prefix and account for the possibility of no space after ":" (#37644)

I'am using an openai compatible model, but got nothing in agent thread
panel, and Zed log has "Model generated an empty summary" line.

I add one log to open_ai.rs:
<img width="2454" height="626" alt="图片"
src="https://github.com/user-attachments/assets/85354c7d-a0cc-4bba-86fd-2a640038a13e"
/>

and got:

<img width="3456" height="278" alt="图片"
src="https://github.com/user-attachments/assets/7746aedd-5d76-44b5-90f2-e129a1507178"
/>

It appear that `let line = line.strip_prefix("data: ")?;` can not handle
correctly.

Release Notes:

- N/A

---------

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>

crates/open_ai/src/open_ai.rs

@@ -473,7 +473,7 @@ pub async fn stream_completion(
             .filter_map(|line| async move {
                 match line {
                     Ok(line) => {
-                        let line = line.strip_prefix("data: ")?;
+                        let line = line.strip_prefix("data: ").or_else(|| line.strip_prefix("data:"))?;
                         if line == "[DONE]" {
                             None
                         } else {