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/protocol/src/protocol.rs

@@ -27,6 +27,13 @@ use crate::models::ResponseItem;
 use crate::parse_command::ParsedCommand;
 use crate::plan_tool::UpdatePlanArgs;
 
+/// Open/close tags for special user-input blocks. Used across crates to avoid
+/// duplicated hardcoded strings.
+pub const USER_INSTRUCTIONS_OPEN_TAG: &str = "<user_instructions>";
+pub const USER_INSTRUCTIONS_CLOSE_TAG: &str = "</user_instructions>";
+pub const ENVIRONMENT_CONTEXT_OPEN_TAG: &str = "<environment_context>";
+pub const ENVIRONMENT_CONTEXT_CLOSE_TAG: &str = "</environment_context>";
+
 /// Submission Queue Entry - requests from user
 #[derive(Debug, Clone, Deserialize, Serialize)]
 pub struct Submission {
@@ -417,6 +424,9 @@ pub enum EventMsg {
     /// Agent text output message
     AgentMessage(AgentMessageEvent),
 
+    /// User/system input message (what was sent to the model)
+    UserMessage(UserMessageEvent),
+
     /// Agent text output delta message
     AgentMessageDelta(AgentMessageDeltaEvent),
 
@@ -610,6 +620,47 @@ pub struct AgentMessageEvent {
     pub message: String,
 }
 
+#[derive(Debug, Clone, Deserialize, Serialize)]
+#[serde(rename_all = "snake_case")]
+pub enum InputMessageKind {
+    /// Plain user text (default)
+    Plain,
+    /// XML-wrapped user instructions (<user_instructions>...)
+    UserInstructions,
+    /// XML-wrapped environment context (<environment_context>...)
+    EnvironmentContext,
+}
+
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct UserMessageEvent {
+    pub message: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub kind: Option<InputMessageKind>,
+}
+
+impl<T, U> From<(T, U)> for InputMessageKind
+where
+    T: AsRef<str>,
+    U: AsRef<str>,
+{
+    fn from(value: (T, U)) -> Self {
+        let (_role, message) = value;
+        let message = message.as_ref();
+        let trimmed = message.trim();
+        if trimmed.starts_with(ENVIRONMENT_CONTEXT_OPEN_TAG)
+            && trimmed.ends_with(ENVIRONMENT_CONTEXT_CLOSE_TAG)
+        {
+            InputMessageKind::EnvironmentContext
+        } else if trimmed.starts_with(USER_INSTRUCTIONS_OPEN_TAG)
+            && trimmed.ends_with(USER_INSTRUCTIONS_CLOSE_TAG)
+        {
+            InputMessageKind::UserInstructions
+        } else {
+            InputMessageKind::Plain
+        }
+    }
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize)]
 pub struct AgentMessageDeltaEvent {
     pub delta: String,