fix: address Copilot review round 21 — seed a nested object's own union branch

collectSchemaDefaults recursed only when a nested object declared `properties`
directly, so one keeping its fields on a composition branch was skipped — while
SchemaForm rendered that branch and displayed its read-only discriminator. The
constant was shown and never submitted, and the server rejected the call. The
recursion now gates on declaresAnyFields.

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

clients/web/src/utils/jsonUtils.test.ts

@@ -457,6 +457,37 @@ describe("root composition (#2123)", () => {
     expect(Object.hasOwn(seeded, "__proto__")).toBe(true);
   });
 
+  it("seeds defaults from a nested object's own union branch", () => {
+    // The nested form renders that branch, so its read-only discriminator has
+    // to reach the submitted values too.
+    expect(
+      collectSchemaDefaults({
+        type: "object",
+        properties: {
+          config: {
+            type: "object",
+            anyOf: [
+              {
+                type: "object",
+                properties: {
+                  kind: { type: "string", const: "email" },
+                  address: { type: "string" },
+                },
+              },
+              {
+                type: "object",
+                properties: {
+                  kind: { type: "string", const: "sms" },
+                  phone: { type: "string" },
+                },
+              },
+            ],
+          },
+        },
+      }),
+    ).toEqual({ config: { kind: "email" } });
+  });
+
   it("re-applies a nested object's constants", () => {
     // The overlay replaces the whole nested object rather than merging into it,
     // so a link naming `{ config: { kind: "sms" } }` would otherwise slip past

clients/web/src/utils/jsonUtils.ts

@@ -4,6 +4,7 @@ import {
 } from "@inspector/core/json/nullableUnion.js";
 import {
   branchAcceptsValues,
+  declaresAnyFields,
   resolveRootUnion,
   selectBranchIndex,
 } from "@inspector/core/json/rootUnion.js";
@@ -176,7 +177,14 @@ export function collectSchemaDefaults(
       seed(fieldName, fieldSchema.const);
     } else if (fieldSchema.default !== undefined) {
       seed(fieldName, fieldSchema.default);
-    } else if (fieldSchema.type === "object" && fieldSchema.properties) {
+    } else if (
+      fieldSchema.type === "object" &&
+      // Not `fieldSchema.properties`: a nested object can keep its fields on a
+      // composition branch too, and `SchemaForm` renders that branch — so
+      // skipping it here would leave its read-only discriminator displayed but
+      // never submitted, and the server would reject the call (#2123).
+      declaresAnyFields(fieldSchema)
+    ) {
       const nested = collectSchemaDefaults(fieldSchema);
       if (Object.keys(nested).length > 0) {
         seed(fieldName, nested);