feat: make ConstraintError an enum (#8330)

This will make it easier to test for expected errors in unit tests since
we can compare based on the field values rather than the message (which
might change over time). See https://github.com/openai/codex/pull/8298
for an example.

It also ensures more consistency in the way a `ConstraintError` is
constructed.

codex-rs/core/src/codex.rs

@@ -78,7 +78,6 @@ use crate::client_common::ResponseEvent;
 use crate::compact::collect_user_messages;
 use crate::config::Config;
 use crate::config::Constrained;
-use crate::config::ConstraintError;
 use crate::config::ConstraintResult;
 use crate::config::GhostSnapshotConfig;
 use crate::config::types::ShellEnvironmentPolicy;
@@ -836,11 +835,8 @@ impl Session {
                 Ok(())
             }
             Err(err) => {
-                let wrapped = ConstraintError {
-                    message: format!("Could not update config: {err}"),
-                };
-                warn!(%wrapped, "rejected session settings update");
-                Err(wrapped)
+                warn!("rejected session settings update: {err}");
+                Err(err)
             }
         }
     }
@@ -861,18 +857,15 @@ impl Session {
                 }
                 Err(err) => {
                     drop(state);
-                    let wrapped = ConstraintError {
-                        message: format!("Could not update config: {err}"),
-                    };
                     self.send_event_raw(Event {
                         id: sub_id.clone(),
                         msg: EventMsg::Error(ErrorEvent {
-                            message: wrapped.to_string(),
+                            message: err.to_string(),
                             codex_error_info: Some(CodexErrorInfo::BadRequest),
                         }),
                     })
                     .await;
-                    return Err(wrapped);
+                    return Err(err);
                 }
             }
         };