[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/tests/common/responses.rs

@@ -296,6 +296,7 @@ pub fn ev_reasoning_summary_text_delta(delta: &str) -> Value {
     serde_json::json!({
         "type": "response.reasoning_summary_text.delta",
         "delta": delta,
+        "summary_index": 0,
     })
 }
 
@@ -303,6 +304,7 @@ pub fn ev_reasoning_text_delta(delta: &str) -> Value {
     serde_json::json!({
         "type": "response.reasoning_text.delta",
         "delta": delta,
+        "content_index": 0,
     })
 }