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/mcp-server/src/outgoing_message.rs

@@ -260,6 +260,7 @@ mod tests {
                 model: "gpt-4o".to_string(),
                 history_log_id: 1,
                 history_entry_count: 1000,
+                initial_messages: None,
             }),
         };
 
@@ -289,6 +290,7 @@ mod tests {
             model: "gpt-4o".to_string(),
             history_log_id: 1,
             history_entry_count: 1000,
+            initial_messages: None,
         };
         let event = Event {
             id: "1".to_string(),