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/core/src/protocol.rs

@@ -235,8 +235,7 @@ impl SandboxPolicy {
         }
     }
 
-    /// Always returns `true` for now, as we do not yet support restricting read
-    /// access.
+    /// Always returns `true`; restricting read access is not supported.
     pub fn has_full_disk_read_access(&self) -> bool {
         true
     }
@@ -384,6 +383,8 @@ pub enum EventMsg {
 
     /// Agent reasoning content delta event from agent.
     AgentReasoningRawContentDelta(AgentReasoningRawContentDeltaEvent),
+    /// Signaled when the model begins a new reasoning summary section (e.g., a new titled block).
+    AgentReasoningSectionBreak(AgentReasoningSectionBreakEvent),
 
     /// Ack the client's configure message.
     SessionConfigured(SessionConfiguredEvent),
@@ -531,6 +532,9 @@ pub struct AgentReasoningRawContentDeltaEvent {
     pub delta: String,
 }
 
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct AgentReasoningSectionBreakEvent {}
+
 #[derive(Debug, Clone, Deserialize, Serialize)]
 pub struct AgentReasoningDeltaEvent {
     pub delta: String,