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/mcp-server/src/codex_tool_runner.rs

@@ -279,6 +279,7 @@ async fn run_codex_tool_session_inner(
                     | EventMsg::PlanUpdate(_)
                     | EventMsg::TurnAborted(_)
                     | EventMsg::ConversationHistory(_)
+                    | EventMsg::UserMessage(_)
                     | EventMsg::ShutdownComplete => {
                         // For now, we do not do anything extra for these
                         // events. Note that