fix(openai): correctly parse Responses API tool call events

- The Responses API does not use 'response.item.delta' for tool calls.
- It uses 'response.output_item.added' to initialize the function call.
- It uses 'response.function_call_arguments.delta' for the payload stream.
- Updated the streaming parser to catch these events and correctly yield ToolCallDelta objects.
- This restores proper streaming of tool calls back to the client.

src/providers/openai.rs

@@ -524,6 +524,7 @@ impl super::Provider for OpenAIProvider {
                         }
                     }
                     Ok(_) => continue,
+                    Err(reqwest_eventsource::Error::StreamEnded) => break,
                     Err(e) => {
                         // Attempt to probe for the actual error body
                         let probe_resp = probe_client
@@ -756,15 +757,11 @@ impl super::Provider for OpenAIProvider {
                                         content_buffer.push_str(delta);
                                     }
                                 }
-                                "response.item.delta" => {
-                                    if let Some(delta) = chunk.get("delta") {
-                                        let t = delta.get("type").and_then(|v| v.as_str()).unwrap_or("");
-                                        if t == "function_call" {
-                                            let call_id = delta.get("call_id")
-                                                .or_else(|| chunk.get("item_id"))
-                                                .and_then(|v| v.as_str());
-                                            let name = delta.get("name").and_then(|v| v.as_str());
-                                            let arguments = delta.get("arguments").and_then(|v| v.as_str());
+                                "response.output_item.added" => {
+                                    if let Some(item) = chunk.get("item") {
+                                        if item.get("type").and_then(|v| v.as_str()) == Some("function_call") {
+                                            let call_id = item.get("call_id").and_then(|v| v.as_str());
+                                            let name = item.get("name").and_then(|v| v.as_str());
                                             
                                             tool_calls = Some(vec![crate::models::ToolCallDelta {
                                                 index: chunk.get("output_index").and_then(|v| v.as_u64()).unwrap_or(0) as u32,
@@ -772,17 +769,26 @@ impl super::Provider for OpenAIProvider {
                                                 call_type: Some("function".to_string()),
                                                 function: Some(crate::models::FunctionCallDelta {
                                                     name: name.map(|s| s.to_string()),
-                                                    arguments: arguments.map(|s| s.to_string()),
+                                                    arguments: Some("".to_string()), // Start with empty arguments
                                                 }),
                                             }]);
-                                        } else if t == "message" {
-                                            if let Some(text) = delta.get("text").and_then(|v| v.as_str()) {
-                                                content_buffer.push_str(text);
                                         }
                                     }
                                 }
+                                "response.function_call_arguments.delta" => {
+                                    if let Some(delta) = chunk.get("delta").and_then(|v| v.as_str()) {
+                                        tool_calls = Some(vec![crate::models::ToolCallDelta {
+                                            index: chunk.get("output_index").and_then(|v| v.as_u64()).unwrap_or(0) as u32,
+                                            id: None,
+                                            call_type: None,
+                                            function: Some(crate::models::FunctionCallDelta {
+                                                name: None,
+                                                arguments: Some(delta.to_string()),
+                                            }),
+                                        }]);
                                     }
-                                "response.output_text.done" | "response.item.done" | "response.done" => {
+                                }
+                                "response.output_text.done" | "response.item.done" | "response.completed" => {
                                     finish_reason = Some("stop".to_string());
                                 }
                                 _ => {}
@@ -887,13 +893,20 @@ impl super::Provider for OpenAIProvider {
                         }
                     }
                     Ok(_) => continue,
+                    Err(reqwest_eventsource::Error::StreamEnded) => break,
                     Err(e) => {
+                        tracing::error!("Responses stream encountered an error: {}", e);
                          // Attempt to probe for the actual error body
+                        let mut probe_body_no_stream = probe_body.clone();
+                        if let Some(obj) = probe_body_no_stream.as_object_mut() {
+                            obj.remove("stream");
+                        }
+
                         let probe_resp = probe_client
                             .post(&url)
                             .header("Authorization", format!("Bearer {}", api_key))
                             .header("Accept", "application/json")
-                            .json(&probe_body)
+                            .json(&probe_body_no_stream)
                             .send()
                             .await;