Replace config.responses_originator_header_internal_override with CODEX_INTERNAL_ORIGINATOR_OVERRIDE_ENV_VAR (#3388)

The previous config approach had a few issues:
1. It is part of the config but not designed to be used externally
2. It had to be wired through many places (look at the +/- on this PR
3. It wasn't guaranteed to be set consistently everywhere because we
don't have a super well defined way that configs stack. For example, the
extension would configure during newConversation but anything that
happened outside of that (like login) wouldn't get it.

This env var approach is cleaner and also creates one less thing we have
to deal with when coming up with a better holistic story around configs.

One downside is that I removed the unit test testing for the override
because I don't want to deal with setting the global env or spawning
child processes and figuring out how to introspect their originator
header. The new code is sufficiently simple and I tested it e2e that I
feel as if this is still worth it.

codex-rs/exec/src/lib.rs

@@ -190,7 +190,6 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
     let conversation_manager = ConversationManager::new(AuthManager::shared(
         config.codex_home.clone(),
         config.preferred_auth_method,
-        config.responses_originator_header.clone(),
     ));
     let NewConversation {
         conversation_id: _,