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/exec/src/event_processor_with_human_output.rs

@@ -552,6 +552,7 @@ impl EventProcessor for EventProcessorWithHumanOutput {
             },
             EventMsg::ShutdownComplete => return CodexStatus::Shutdown,
             EventMsg::ConversationHistory(_) => {}
+            EventMsg::UserMessage(_) => {}
         }
         CodexStatus::Running
     }