[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/protocol/src/protocol.rs

@@ -617,6 +617,9 @@ pub struct ReasoningContentDeltaEvent {
     pub turn_id: String,
     pub item_id: String,
     pub delta: String,
+    // load with default value so it's backward compatible with the old format.
+    #[serde(default)]
+    pub summary_index: i64,
 }
 
 impl HasLegacyEvent for ReasoningContentDeltaEvent {
@@ -633,6 +636,9 @@ pub struct ReasoningRawContentDeltaEvent {
     pub turn_id: String,
     pub item_id: String,
     pub delta: String,
+    // load with default value so it's backward compatible with the old format.
+    #[serde(default)]
+    pub content_index: i64,
 }
 
 impl HasLegacyEvent for ReasoningRawContentDeltaEvent {
@@ -923,7 +929,13 @@ pub struct AgentReasoningRawContentDeltaEvent {
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
-pub struct AgentReasoningSectionBreakEvent {}
+pub struct AgentReasoningSectionBreakEvent {
+    // load with default value so it's backward compatible with the old format.
+    #[serde(default)]
+    pub item_id: String,
+    #[serde(default)]
+    pub summary_index: i64,
+}
 
 #[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
 pub struct AgentReasoningDeltaEvent {