fix: address Copilot review round 13 — cross-keyword contradictions, own-property required check

- rootUnion: keywords the two declarations state SEPARATELY can contradict too.
  A merged `const` is now checked against its own `type` and `enum`, so
  root `x: { type: "string" }` under branch `x: { const: 1 }` declines
  instead of seeding an immutable 1 into a string field. An integer const still
  satisfies a `number` type — the one direction JSON Schema widens.
- TUI missingRequiredFields tests `Object.hasOwn` first: an argument legally
  named `constructor` resolved to the inherited one and read as supplied, and
  the call went out without it.

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

clients/tui/__tests__/schemaToForm.test.ts

@@ -808,6 +808,17 @@ describe("schemaToForm", () => {
       ).toEqual([]);
     });
 
+    it("does not mistake an inherited property for a supplied argument", () => {
+      // `constructor` is a legal argument name; reading it off the prototype
+      // would report it as present and send the call without it.
+      const schema = {
+        type: "object",
+        properties: { constructor: { type: "string" } },
+        required: ["constructor"],
+      };
+      expect(missingRequiredFields(schema, {})).toEqual(["constructor"]);
+    });
+
     it("checks the root's own required fields when there is no union", () => {
       const schema = {
         type: "object",