save request payload on any API error, not just timeouts

Serialize request JSON before send_and_check so it's available
for both HTTP errors and stream errors. Extracted save logic
into save_failed_request helper on SseReader.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/api/openai.rs

@@ -55,6 +55,7 @@ pub async fn stream_events(
         None => String::new(),
     };
     let debug_label = format!("{} messages, model={}{}", msg_count, model, pri_label);
+    let request_json = serde_json::to_string_pretty(&request).ok();
 
     let mut response = super::send_and_check(
         client,
@@ -64,11 +65,12 @@ pub async fn stream_events(
         &[],
         ui_tx,
         &debug_label,
+        request_json.as_deref(),
     )
     .await?;
 
     let mut reader = super::SseReader::new(ui_tx);
-    reader.set_request(&request);
+    reader.request_json = request_json;
 
     let mut content_len: usize = 0;
     let mut reasoning_chars: usize = 0;