Replay EventMsgs from Response Items when resuming a session with history. (#3123)

### Overview

This PR introduces the following changes:
	1.	Adds a unified mechanism to convert ResponseItem into EventMsg.
2. Ensures that when a session is initialized with initial history, a
vector of EventMsg is sent along with the session configuration. This
allows clients to re-render the UI accordingly.
	3. 	Added integration testing

### Caveats

This implementation does not send every EventMsg that was previously
dispatched to clients. The excluded events fall into two categories:
	•	“Arguably” rolled-out events
Examples include tool calls and apply-patch calls. While these events
are conceptually rolled out, we currently only roll out ResponseItems.
These events are already being handled elsewhere and transformed into
EventMsg before being sent.
	•	Non-rolled-out events
Certain events such as TurnDiff, Error, and TokenCount are not rolled
out at all.

### Future Directions

At present, resuming a session involves maintaining two states:
	•	UI State
Clients can replay most of the important UI from the provided EventMsg
history.
	•	Model State
The model receives the complete session history to reconstruct its
internal state.

This design provides a solid foundation. If, in the future, more precise
UI reconstruction is needed, we have two potential paths:
1. Introduce a third data structure that allows us to derive both
ResponseItems and EventMsgs.
2. Clearly divide responsibilities: the core system ensures the
integrity of the model state, while clients are responsible for
reconstructing the UI.

codex-rs/core/src/event_mapping.rs

@@ -0,0 +1,74 @@
+use crate::protocol::AgentMessageEvent;
+use crate::protocol::AgentReasoningEvent;
+use crate::protocol::AgentReasoningRawContentEvent;
+use crate::protocol::EventMsg;
+use crate::protocol::WebSearchEndEvent;
+use codex_protocol::models::ContentItem;
+use codex_protocol::models::ReasoningItemContent;
+use codex_protocol::models::ReasoningItemReasoningSummary;
+use codex_protocol::models::ResponseItem;
+use codex_protocol::models::WebSearchAction;
+
+/// Convert a `ResponseItem` into zero or more `EventMsg` values that the UI can render.
+///
+/// When `show_raw_agent_reasoning` is false, raw reasoning content events are omitted.
+pub(crate) fn map_response_item_to_event_messages(
+    item: &ResponseItem,
+    show_raw_agent_reasoning: bool,
+) -> Vec<EventMsg> {
+    match item {
+        ResponseItem::Message { content, .. } => {
+            let mut events = Vec::new();
+            for content_item in content {
+                if let ContentItem::OutputText { text } = content_item {
+                    events.push(EventMsg::AgentMessage(AgentMessageEvent {
+                        message: text.clone(),
+                    }));
+                }
+            }
+            events
+        }
+
+        ResponseItem::Reasoning {
+            summary, content, ..
+        } => {
+            let mut events = Vec::new();
+            for ReasoningItemReasoningSummary::SummaryText { text } in summary {
+                events.push(EventMsg::AgentReasoning(AgentReasoningEvent {
+                    text: text.clone(),
+                }));
+            }
+            if let Some(items) = content.as_ref().filter(|_| show_raw_agent_reasoning) {
+                for c in items {
+                    let text = match c {
+                        ReasoningItemContent::ReasoningText { text }
+                        | ReasoningItemContent::Text { text } => text,
+                    };
+                    events.push(EventMsg::AgentReasoningRawContent(
+                        AgentReasoningRawContentEvent { text: text.clone() },
+                    ));
+                }
+            }
+            events
+        }
+
+        ResponseItem::WebSearchCall { id, action, .. } => match action {
+            WebSearchAction::Search { query } => {
+                let call_id = id.clone().unwrap_or_else(|| "".to_string());
+                vec![EventMsg::WebSearchEnd(WebSearchEndEvent {
+                    call_id,
+                    query: query.clone(),
+                })]
+            }
+            WebSearchAction::Other => Vec::new(),
+        },
+
+        // Variants that require side effects are handled by higher layers and do not emit events here.
+        ResponseItem::FunctionCall { .. }
+        | ResponseItem::FunctionCallOutput { .. }
+        | ResponseItem::LocalShellCall { .. }
+        | ResponseItem::CustomToolCall { .. }
+        | ResponseItem::CustomToolCallOutput { .. }
+        | ResponseItem::Other => Vec::new(),
+    }
+}