fix: address Copilot review round 32 — normalize before the type vote, enum-spelled null

- coercionProperties collapses each declaration before comparing types. A
  nullable schema written as an `anyOf` states no top-level `type`, so
  `number | null` and `boolean | null` both read as "no type" and counted
  as agreeing — and `value=true` then came back as `NaN` through the first
  branch's number.
- admitsNull reads an `enum` offering null, under the same sibling conditions
  a `const: null` answers to. Without it `{ enum: [null] }` reported that it
  rejects null, and a required read-only field spelled that way stayed
  permanently invalid.

Signed-off-by: cliffhall <cliff@futurescale.com>

core/json/nullableUnion.ts

@@ -406,6 +406,16 @@ export function admitsNull(schema: NullableUnionSchema): boolean {
     return schema.anyOf === undefined || anyOfAdmitsNull(schema);
   }
 
+  // An `enum` offering `null` admits it, the same way a `const: null` does —
+  // and under the same sibling conditions, since the two are equally
+  // conjunctive with whatever else the schema states.
+  if (Array.isArray(schema.enum) && schema.enum.includes(null)) {
+    return (
+      (schema.nullable === true || typeAdmitsNull(schema.type)) &&
+      (schema.anyOf === undefined || anyOfAdmitsNull(schema))
+    );
+  }
+
   if (schema.nullable === true) {
     return true;
   }