Add new thread items and rewire event parsing to use them (#5418)

1. Adds AgentMessage,  Reasoning,  WebSearch items.
2. Switches the ResponseItem parsing to use new items and then also emit
3. Removes user-item kind and filters out "special" (environment) user
items when returning to clients.

codex-rs/core/src/chat_completions.rs

@@ -104,10 +104,10 @@ pub(crate) async fn stream_chat_completions(
             } = item
             {
                 let mut text = String::new();
-                for c in items {
-                    match c {
-                        ReasoningItemContent::ReasoningText { text: t }
-                        | ReasoningItemContent::Text { text: t } => text.push_str(t),
+                for entry in items {
+                    match entry {
+                        ReasoningItemContent::ReasoningText { text: segment }
+                        | ReasoningItemContent::Text { text: segment } => text.push_str(segment),
                     }
                 }
                 if text.trim().is_empty() {