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/config/constraint.rs

@@ -4,25 +4,25 @@ use std::sync::Arc;
 use thiserror::Error;
 
 #[derive(Debug, Error, PartialEq, Eq)]
-#[error("{message}")]
-pub struct ConstraintError {
-    pub message: String,
+pub enum ConstraintError {
+    #[error("value `{candidate}` is not in the allowed set {allowed}")]
+    InvalidValue { candidate: String, allowed: String },
+
+    #[error("field `{field_name}` cannot be empty")]
+    EmptyField { field_name: String },
 }
 
 impl ConstraintError {
     pub fn invalid_value(candidate: impl Into<String>, allowed: impl Into<String>) -> Self {
-        Self {
-            message: format!(
-                "value `{}` is not in the allowed set {}",
-                candidate.into(),
-                allowed.into()
-            ),
+        Self::InvalidValue {
+            candidate: candidate.into(),
+            allowed: allowed.into(),
         }
     }
 
     pub fn empty_field(field_name: impl Into<String>) -> Self {
-        Self {
-            message: format!("field `{}` cannot be empty", field_name.into()),
+        Self::EmptyField {
+            field_name: field_name.into(),
         }
     }
 }