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/tui/src/chatwidget/tests.rs

@@ -23,7 +23,6 @@ use codex_core::protocol::ExecCommandBeginEvent;
 use codex_core::protocol::ExecCommandEndEvent;
 use codex_core::protocol::ExitedReviewModeEvent;
 use codex_core::protocol::FileChange;
-use codex_core::protocol::InputMessageKind;
 use codex_core::protocol::Op;
 use codex_core::protocol::PatchApplyBeginEvent;
 use codex_core::protocol::PatchApplyEndEvent;
@@ -104,7 +103,6 @@ fn resumed_initial_messages_render_history() {
         initial_messages: Some(vec![
             EventMsg::UserMessage(UserMessageEvent {
                 message: "hello from user".to_string(),
-                kind: Some(InputMessageKind::Plain),
                 images: None,
             }),
             EventMsg::AgentMessage(AgentMessageEvent {