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/protocol/src/protocol.rs

@@ -839,6 +839,11 @@ pub struct SessionConfiguredEvent {
 
     /// Current number of entries in the history log.
     pub history_entry_count: usize,
+
+    /// Optional initial messages (as events) for resumed sessions.
+    /// When present, UIs can use these to seed the history.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub initial_messages: Option<Vec<EventMsg>>,
 }
 
 /// User's decision in response to an ExecApprovalRequest.
@@ -914,6 +919,7 @@ mod tests {
                 model: "codex-mini-latest".to_string(),
                 history_log_id: 0,
                 history_entry_count: 0,
+                initial_messages: None,
             }),
         };
         let serialized = serde_json::to_string(&event).unwrap();