Dividing UserMsgs into categories to send it back to the tui (#3127)

This PR does the following:

- divides user msgs into 3 categories: plain, user instructions, and
environment context
- Centralizes adding user instructions and environment context to a
degree
- Improve the integration testing

Building on top of #3123

Specifically this
[comment](https://github.com/openai/codex/pull/3123#discussion_r2319885089).
We need to send the user message while ignoring the User Instructions
and Environment Context we attach.

codex-rs/core/src/event_mapping.rs

@@ -2,6 +2,8 @@ use crate::protocol::AgentMessageEvent;
 use crate::protocol::AgentReasoningEvent;
 use crate::protocol::AgentReasoningRawContentEvent;
 use crate::protocol::EventMsg;
+use crate::protocol::InputMessageKind;
+use crate::protocol::UserMessageEvent;
 use crate::protocol::WebSearchEndEvent;
 use codex_protocol::models::ContentItem;
 use codex_protocol::models::ReasoningItemContent;
@@ -17,15 +19,37 @@ pub(crate) fn map_response_item_to_event_messages(
     show_raw_agent_reasoning: bool,
 ) -> Vec<EventMsg> {
     match item {
-        ResponseItem::Message { content, .. } => {
-            let mut events = Vec::new();
-            for content_item in content {
-                if let ContentItem::OutputText { text } = content_item {
-                    events.push(EventMsg::AgentMessage(AgentMessageEvent {
-                        message: text.clone(),
-                    }));
-                }
+        ResponseItem::Message { role, content, .. } => {
+            // Do not surface system messages as user events.
+            if role == "system" {
+                return Vec::new();
             }
+
+            let events: Vec<EventMsg> = content
+                .iter()
+                .filter_map(|content_item| match content_item {
+                    ContentItem::OutputText { text } => {
+                        Some(EventMsg::AgentMessage(AgentMessageEvent {
+                            message: text.clone(),
+                        }))
+                    }
+                    ContentItem::InputText { text } => {
+                        let trimmed = text.trim_start();
+                        let kind = if trimmed.starts_with("<environment_context>") {
+                            Some(InputMessageKind::EnvironmentContext)
+                        } else if trimmed.starts_with("<user_instructions>") {
+                            Some(InputMessageKind::UserInstructions)
+                        } else {
+                            Some(InputMessageKind::Plain)
+                        };
+                        Some(EventMsg::UserMessage(UserMessageEvent {
+                            message: text.clone(),
+                            kind,
+                        }))
+                    }
+                    _ => None,
+                })
+                .collect();
             events
         }