fork conversation from a previous message (#2575)

This can be the underlying logic in order to start a conversation from a
previous message. will need some love in the UI.

Base for building this: #2588

codex-rs/mcp-server/src/codex_tool_runner.rs

@@ -275,6 +275,7 @@ async fn run_codex_tool_session_inner(
                     | EventMsg::GetHistoryEntryResponse(_)
                     | EventMsg::PlanUpdate(_)
                     | EventMsg::TurnAborted(_)
+                    | EventMsg::ConversationHistory(_)
                     | EventMsg::ShutdownComplete => {
                         // For now, we do not do anything extra for these
                         // events. Note that