fix: return thinking as reasoning output item per OpenAI spec

Thinking content was previously returned as non-standard top-level
fields (thinking, thinking_duration). Now follows the official OpenAI
Responses API format:

- Reasoning appears as a 'type: reasoning' item in the output array
  with summary[].text containing the thinking content
- Message item follows after the reasoning item
- thinking_signature kept as proxy extension (internal multi-turn data)
- Removed ResponseOutput/OutputContent structs in favor of
  serde_json::Value for polymorphic output items

src/api/responses.rs

@@ -72,11 +72,9 @@ struct ResponseData {
     status: &'static str,
     created_at: u64,
     completed_at: Option<u64>,
-    output: Vec<ResponseOutput>,
+    output: Vec<serde_json::Value>,
     usage: Option<Usage>,
     thinking_signature: Option<String>,
-    thinking: Option<String>,
-    thinking_duration: Option<String>,
 }
 
 /// Build a full Response object matching the official OpenAI schema.
@@ -107,8 +105,6 @@ fn build_response_object(data: ResponseData, params: &RequestParams) -> Response
         user: params.user.clone(),
         metadata: params.metadata.clone(),
         thinking_signature: data.thinking_signature,
-        thinking: data.thinking,
-        thinking_duration: data.thinking_duration,
     }
 }
 
@@ -339,6 +335,13 @@ async fn handle_responses_sync(
 
     let usage = usage_from_poll(&state.mitm_store, &cascade_id, &poll_result.usage, &params.user_text, &poll_result.text).await;
 
+    // Build output array: [reasoning (if present), message]
+    let mut output_items: Vec<serde_json::Value> = Vec::new();
+    if let Some(ref thinking_text) = poll_result.thinking {
+        output_items.push(build_reasoning_output(thinking_text));
+    }
+    output_items.push(build_message_output(&msg_id, &poll_result.text));
+
     let resp = build_response_object(
         ResponseData {
             id: response_id,
@@ -346,21 +349,9 @@ async fn handle_responses_sync(
             status: "completed",
             created_at,
             completed_at: Some(completed_at),
-            output: vec![ResponseOutput {
-                output_type: "message",
-                id: msg_id,
-                status: "completed",
-                role: "assistant",
-                content: vec![OutputContent {
-                    content_type: "output_text",
-                    text: poll_result.text,
-                    annotations: vec![],
-                }],
-            }],
+            output: output_items,
             usage: Some(usage),
             thinking_signature: poll_result.thinking_signature,
-            thinking: poll_result.thinking,
-            thinking_duration: poll_result.thinking_duration,
         },
         &params,
     );
@@ -397,8 +388,6 @@ async fn handle_responses_stream(
                 output: vec![],
                 usage: None,
                 thinking_signature: None,
-                thinking: None,
-                thinking_duration: None,
             },
             &params,
         );
@@ -424,20 +413,14 @@ async fn handle_responses_stream(
             }),
         ));
 
-        // 3. response.output_item.added
+        // 3. response.output_item.added (message — reasoning added at completion)
         yield Ok(responses_sse_event(
             "response.output_item.added",
             serde_json::json!({
                 "type": "response.output_item.added",
                 "sequence_number": next_seq(),
                 "output_index": OUTPUT_IDX,
-                "item": {
-                    "type": "message",
-                    "id": &msg_id,
-                    "status": "in_progress",
-                    "role": "assistant",
-                    "content": [],
-                }
+                "item": build_message_output_in_progress(&msg_id),
             }),
         ));
 
@@ -554,8 +537,6 @@ async fn handle_responses_stream(
                 output: vec![],
                 usage: Some(Usage::estimate(&params.user_text, "")),
                 thinking_signature: None,
-                thinking: None,
-                thinking_duration: None,
             },
             &params,
         );
@@ -584,7 +565,7 @@ async fn handle_responses_stream(
 /// 1. response.output_text.done
 /// 2. response.content_part.done
 /// 3. response.output_item.done
-/// 4. response.completed
+/// 4. response.completed (with reasoning item prepended if present)
 #[allow(clippy::too_many_arguments)]
 fn completion_events(
     resp_id: &str,
@@ -599,22 +580,19 @@ fn completion_events(
     params: &RequestParams,
     thinking_signature: Option<String>,
     thinking: Option<String>,
-    thinking_duration: Option<String>,
+    _thinking_duration: Option<String>,
 ) -> Vec<Event> {
     let next_seq = || seq.fetch_add(1, Ordering::Relaxed);
     let completed_at = now_unix();
 
-    let output_item = serde_json::json!({
-        "type": "message",
-        "id": msg_id,
-        "status": "completed",
-        "role": "assistant",
-        "content": [{
-            "type": "output_text",
-            "text": text,
-            "annotations": [],
-        }],
-    });
+    let output_item = build_message_output(msg_id, text);
+
+    // Build output array: [reasoning (if present), message]
+    let mut output_items: Vec<serde_json::Value> = Vec::new();
+    if let Some(ref thinking_text) = thinking {
+        output_items.push(build_reasoning_output(thinking_text));
+    }
+    output_items.push(build_message_output(msg_id, text));
 
     let completed_resp = build_response_object(
         ResponseData {
@@ -623,21 +601,9 @@ fn completion_events(
             status: "completed",
             created_at,
             completed_at: Some(completed_at),
-            output: vec![ResponseOutput {
-                output_type: "message",
-                id: msg_id.to_string(),
-                status: "completed",
-                role: "assistant",
-                content: vec![OutputContent {
-                    content_type: "output_text",
-                    text: text.to_string(),
-                    annotations: vec![],
-                }],
-            }],
+            output: output_items,
             usage: Some(usage),
             thinking_signature,
-            thinking,
-            thinking_duration,
         },
         params,
     );