fix: address Copilot review round 4 — allOf faithfulness, const null, branch-aware seeding

- rootUnion: an `allOf` member is foldable only when it states nothing beyond
  what the merge applies (`type`/`properties`/`required` plus annotations),
  so a nested `anyOf`, a `not`, or an `additionalProperties` can no longer
  be erased along with the keyword; members are checked against the
  ACCUMULATED merge, so two of them contradicting each other is caught even
  when neither contradicts the root.
- admitsNull: `const: null` admits null, so a required field pinned to it is
  no longer seeded with the one value it accepts and then reported missing.
- collectSchemaDefaults takes the values a caller is about to overlay, and
  seeds the branch THEY identify — the App deep link passes its `appArgs`, so
  branch 0's defaults can no longer sit invisibly under branch 1's arguments.
- The branch inference moves into core as `selectBranchIndex`, shared by the
  seeding and the form's picker so the two cannot disagree.

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

clients/web/src/utils/jsonUtils.ts

@@ -2,7 +2,10 @@ import {
   admitsNull,
   normalizeNullableUnion,
 } from "@inspector/core/json/nullableUnion.js";
-import { resolveRootUnion } from "@inspector/core/json/rootUnion.js";
+import {
+  resolveRootUnion,
+  selectBranchIndex,
+} from "@inspector/core/json/rootUnion.js";
 
 export type JsonValue =
   | string
@@ -124,14 +127,23 @@ export function getDataType(value: JsonValue): DataType {
  */
 export function collectSchemaDefaults(
   schema: InspectorFormSchema,
+  knownValues: Record<string, unknown> = {},
 ): Record<string, unknown> {
   // Seed from the shape the form actually renders: root `allOf` merged in, and
-  // for a root union the branch the picker starts on (#2123). Seeding every
+  // for a root union the branch the picker will open on (#2123). Seeding every
   // branch would put fields of shapes the call is not making into the
   // arguments; seeding none would leave the branch's defaults — its
   // discriminator `const` among them — displayed but never submitted.
+  //
+  // `knownValues` is for a caller that already holds arguments it is about to
+  // overlay on these defaults, as the App deep link does with its `appArgs`.
+  // Those values can name a branch other than the first through its
+  // discriminator, and seeding the first branch's defaults underneath them
+  // would leave another shape's fields in the submitted arguments, invisible
+  // to a form showing the branch the values actually identify.
   const { base, branches } = resolveRootUnion(schema);
-  const properties = (branches[0]?.schema ?? base).properties ?? {};
+  const selected = selectBranchIndex(branches, knownValues) ?? 0;
+  const properties = (branches[selected]?.schema ?? base).properties ?? {};
   const result: Record<string, unknown> = {};
   for (const [fieldName, rawSchema] of Object.entries(properties)) {
     // Collapse a nullable union first, for the same reason `SchemaForm` does: