[App-server] add new v2 events:item/reasoning/delta, item/agentMessage/delta & item/reasoning/summaryPartAdded (#6559)

core event to app server event mapping:
1. `codex/event/reasoning_content_delta` ->
`item/reasoning/summaryTextDelta`.
2. `codex/event/reasoning_raw_content_delta` ->
`item/reasoning/textDelta`
3. `codex/event/agent_message_content_delta` →
`item/agentMessage/delta`.
4. `codex/event/agent_reasoning_section_break` ->
`item/reasoning/summaryPartAdded`.

Also added a change in core to pass down content index, summary index
and item id from events.

Tested with the `git checkout owen/app_server_test_client && cargo run
-p codex-app-server-test-client -- send-message-v2 "hello"` and verified
that new events are emitted correctly.

codex-rs/core/src/codex.rs

@@ -2232,13 +2232,17 @@ async fn try_run_turn(
                     error_or_panic("ReasoningSummaryDelta without active item".to_string());
                 }
             }
-            ResponseEvent::ReasoningSummaryDelta(delta) => {
+            ResponseEvent::ReasoningSummaryDelta {
+                delta,
+                summary_index,
+            } => {
                 if let Some(active) = active_item.as_ref() {
                     let event = ReasoningContentDeltaEvent {
                         thread_id: sess.conversation_id.to_string(),
                         turn_id: turn_context.sub_id.clone(),
                         item_id: active.id(),
-                        delta: delta.clone(),
+                        delta,
+                        summary_index,
                     };
                     sess.send_event(&turn_context, EventMsg::ReasoningContentDelta(event))
                         .await;
@@ -2246,18 +2250,29 @@ async fn try_run_turn(
                     error_or_panic("ReasoningSummaryDelta without active item".to_string());
                 }
             }
-            ResponseEvent::ReasoningSummaryPartAdded => {
-                let event =
-                    EventMsg::AgentReasoningSectionBreak(AgentReasoningSectionBreakEvent {});
-                sess.send_event(&turn_context, event).await;
+            ResponseEvent::ReasoningSummaryPartAdded { summary_index } => {
+                if let Some(active) = active_item.as_ref() {
+                    let event =
+                        EventMsg::AgentReasoningSectionBreak(AgentReasoningSectionBreakEvent {
+                            item_id: active.id(),
+                            summary_index,
+                        });
+                    sess.send_event(&turn_context, event).await;
+                } else {
+                    error_or_panic("ReasoningSummaryPartAdded without active item".to_string());
+                }
             }
-            ResponseEvent::ReasoningContentDelta(delta) => {
+            ResponseEvent::ReasoningContentDelta {
+                delta,
+                content_index,
+            } => {
                 if let Some(active) = active_item.as_ref() {
                     let event = ReasoningRawContentDeltaEvent {
                         thread_id: sess.conversation_id.to_string(),
                         turn_id: turn_context.sub_id.clone(),
                         item_id: active.id(),
-                        delta: delta.clone(),
+                        delta,
+                        content_index,
                     };
                     sess.send_event(&turn_context, EventMsg::ReasoningRawContentDelta(event))
                         .await;