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/tests/chat_completions_sse.rs

@@ -13,6 +13,7 @@ use codex_core::WireApi;
 use codex_core::spawn::CODEX_SANDBOX_NETWORK_DISABLED_ENV_VAR;
 use codex_otel::otel_event_manager::OtelEventManager;
 use codex_protocol::ConversationId;
+use codex_protocol::models::ReasoningItemContent;
 use core_test_support::load_default_config_for_test;
 use futures::StreamExt;
 use tempfile::TempDir;
@@ -143,8 +144,8 @@ fn assert_reasoning(item: &ResponseItem, expected: &str) {
         let mut combined = String::new();
         for part in parts {
             match part {
-                codex_core::ReasoningItemContent::ReasoningText { text }
-                | codex_core::ReasoningItemContent::Text { text } => combined.push_str(text),
+                ReasoningItemContent::ReasoningText { text }
+                | ReasoningItemContent::Text { text } => combined.push_str(text),
             }
         }
         assert_eq!(combined, expected);