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_payload.rs

@@ -8,12 +8,12 @@ use codex_core::LocalShellStatus;
 use codex_core::ModelClient;
 use codex_core::ModelProviderInfo;
 use codex_core::Prompt;
-use codex_core::ReasoningItemContent;
 use codex_core::ResponseItem;
 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 serde_json::Value;