Re-add markdown streaming (#2029)

Wait for newlines, then render markdown on a line by line basis. Word wrap it for the current terminal size and then spit it out line by line into the UI. Also adds tests and fixes some UI regressions.

codex-rs/exec/src/event_processor_with_human_output.rs

@@ -210,6 +210,14 @@ impl EventProcessor for EventProcessorWithHumanOutput {
                 #[allow(clippy::expect_used)]
                 std::io::stdout().flush().expect("could not flush stdout");
             }
+            EventMsg::AgentReasoningSectionBreak(_) => {
+                if !self.show_agent_reasoning {
+                    return CodexStatus::Running;
+                }
+                println!();
+                #[allow(clippy::expect_used)]
+                std::io::stdout().flush().expect("could not flush stdout");
+            }
             EventMsg::AgentReasoningRawContent(AgentReasoningRawContentEvent { text }) => {
                 if !self.show_raw_agent_reasoning {
                     return CodexStatus::Running;