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/models.rs

@@ -309,6 +309,8 @@ impl std::ops::Deref for FunctionCallOutputPayload {
     }
 }
 
+// (Moved event mapping logic into codex-core to avoid coupling protocol to UI-facing events.)
+
 #[cfg(test)]
 mod tests {
     use super::*;