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/environment_context.rs

@@ -8,12 +8,10 @@ use crate::shell::Shell;
 use codex_protocol::config_types::SandboxMode;
 use codex_protocol::models::ContentItem;
 use codex_protocol::models::ResponseItem;
+use codex_protocol::protocol::ENVIRONMENT_CONTEXT_CLOSE_TAG;
+use codex_protocol::protocol::ENVIRONMENT_CONTEXT_OPEN_TAG;
 use std::path::PathBuf;
 
-/// wraps environment context message in a tag for the model to parse more easily.
-pub(crate) const ENVIRONMENT_CONTEXT_START: &str = "<environment_context>";
-pub(crate) const ENVIRONMENT_CONTEXT_END: &str = "</environment_context>";
-
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, DeriveDisplay)]
 #[serde(rename_all = "kebab-case")]
 #[strum(serialize_all = "kebab-case")]
@@ -79,7 +77,7 @@ impl EnvironmentContext {
     /// </environment_context>
     /// ```
     pub fn serialize_to_xml(self) -> String {
-        let mut lines = vec![ENVIRONMENT_CONTEXT_START.to_string()];
+        let mut lines = vec![ENVIRONMENT_CONTEXT_OPEN_TAG.to_string()];
         if let Some(cwd) = self.cwd {
             lines.push(format!("  <cwd>{}</cwd>", cwd.to_string_lossy()));
         }
@@ -101,7 +99,7 @@ impl EnvironmentContext {
         {
             lines.push(format!("  <shell>{shell_name}</shell>"));
         }
-        lines.push(ENVIRONMENT_CONTEXT_END.to_string());
+        lines.push(ENVIRONMENT_CONTEXT_CLOSE_TAG.to_string());
         lines.join("\n")
     }
 }