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/exec/src/event_processor_with_human_output.rs

@@ -515,6 +515,7 @@ impl EventProcessor for EventProcessorWithHumanOutput {
                     model,
                     history_log_id: _,
                     history_entry_count: _,
+                    initial_messages: _,
                 } = session_configured_event;
 
                 ts_println!(