Remove initial messages from session configuration events (#50360)

## What changed

Remove `initial_messages` from `SessionConfiguredEvent` and the corresponding schema. Stop attaching replay history to startup events for resumed or rejoined threads, and remove `InitialHistory::get_event_msgs`.

## Testing

Update the MCP resume test to verify persisted tool events through `load_history`, and retain coverage that resumed requests include prior conversation items.

GitOrigin-RevId: 54b72bc44423c9b1ef2051891ef392c0ede32fe5

codex-rs/protocol/src/protocol.rs

@@ -4007,11 +4007,6 @@ pub struct SessionConfiguredEvent {
     #[serde(skip_serializing_if = "Option::is_none")]
     pub reasoning_effort: Option<ReasoningEffortConfig>,
 
-    /// 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>>,
-
     /// Runtime proxy bind addresses, when the managed proxy was started for this session.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     #[ts(optional)]
@@ -4053,7 +4048,6 @@ impl<'de> Deserialize<'de> for SessionConfiguredEvent {
             active_permission_profile: Option<ActivePermissionProfile>,
             cwd: AbsolutePathBuf,
             reasoning_effort: Option<ReasoningEffortConfig>,
-            initial_messages: Option<Vec<EventMsg>>,
             network_proxy: Option<SessionNetworkProxyRuntime>,
             rollout_path: Option<PathBuf>,
         }
@@ -4086,7 +4080,6 @@ impl<'de> Deserialize<'de> for SessionConfiguredEvent {
             active_permission_profile: wire.active_permission_profile,
             cwd: wire.cwd,
             reasoning_effort: wire.reasoning_effort,
-            initial_messages: wire.initial_messages,
             network_proxy: wire.network_proxy,
             rollout_path: wire.rollout_path,
         })
@@ -6296,7 +6289,6 @@ mod tests {
                 active_permission_profile: None,
                 cwd: test_path_buf("/home/user/project").abs(),
                 reasoning_effort: Some(ReasoningEffortConfig::default()),
-                initial_messages: None,
                 network_proxy: None,
                 rollout_path: Some(rollout_file.path().to_path_buf()),
             }),