api: include turn/payload/message count in API error messages

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

poc-memory/src/agents/api.rs

@@ -59,7 +59,13 @@ pub async fn call_api_with_tools(
             &ui_tx,
             StreamTarget::Autonomous,
             &reasoning,
-        ).await.map_err(|e| format!("API error: {}", e))?;
+        ).await.map_err(|e| {
+            let msg_bytes: usize = messages.iter()
+                .map(|m| m.content_text().len())
+                .sum();
+            format!("API error on turn {} (~{}KB payload, {} messages): {}",
+                turn, msg_bytes / 1024, messages.len(), e)
+        })?;
 
         if let Some(u) = &usage {
             log(&format!("tokens: {} prompt + {} completion",