test(domain): pin audit serde contracts; loosen time version; note null caveat

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/domain/src/audit.rs

@@ -44,6 +44,10 @@ pub enum AuditActor {
 }
 
 /// One field's before/after values within a change.
+///
+/// Note: after a JSON round-trip, `Some(Value::Null)` is indistinguishable from
+/// `None`. Use `None` to mean "no value"; do not encode an absent value as
+/// `Some(Value::Null)`.
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct FieldChange {
     /// Field name (catalogue field key or column name).
@@ -111,10 +115,33 @@ mod tests {
     }
 
     #[test]
-    fn actor_is_adjacently_tagged() {
-        let v = serde_json::to_value(AuditActor::User(Uuid::nil())).unwrap();
-        assert_eq!(v["kind"], "user");
-        let v2 = serde_json::to_value(AuditActor::System).unwrap();
-        assert_eq!(v2["kind"], "system");
+    fn actor_serde_round_trips() {
+        for actor in [AuditActor::User(Uuid::nil()), AuditActor::System] {
+            let v = serde_json::to_value(actor).unwrap();
+            let back: AuditActor = serde_json::from_value(v).unwrap();
+            assert_eq!(back, actor);
+        }
+        assert_eq!(
+            serde_json::to_value(AuditActor::User(Uuid::nil())).unwrap()["kind"],
+            "user"
+        );
+        assert_eq!(
+            serde_json::to_value(AuditActor::System).unwrap()["kind"],
+            "system"
+        );
+    }
+
+    #[test]
+    fn action_serde_matches_as_str() {
+        for a in [
+            AuditAction::Created,
+            AuditAction::Updated,
+            AuditAction::Deleted,
+        ] {
+            assert_eq!(
+                serde_json::to_value(a).unwrap(),
+                serde_json::Value::String(a.as_str().to_owned())
+            );
+        }
     }
 }