fix: address Copilot review round 18 — empty-string constants, nested branch reset

- TUI missingRequiredFields treats an empty string as supplied when the
  property's own `const` pins it there. The one-option control cannot produce
  anything else, so reporting the seeded value as missing made such a branch
  permanently uncallable.
- A nested SchemaForm is reset by the branch-aware key too. Two outer
  alternatives can both carry the same nested object field, and a nested form
  left mounted across an outer switch kept displaying the inner branch chosen
  for the other one while the newly seeded values described another shape.

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

clients/web/src/components/groups/SchemaForm/SchemaForm.test.tsx

@@ -2215,6 +2215,51 @@ describe("SchemaForm multiline strings (#2042)", () => {
       );
     });
 
+    it("resets a nested form's own branch when the outer branch changes", async () => {
+      const user = userEvent.setup();
+      const nested: InspectorFormSchema = {
+        type: "object",
+        properties: {
+          config: {
+            type: "object",
+            title: "Config",
+            properties: {},
+            anyOf: [
+              {
+                type: "object",
+                title: "Inner A",
+                properties: { alpha: { type: "string", title: "Alpha" } },
+              },
+              {
+                type: "object",
+                title: "Inner B",
+                properties: { beta: { type: "string", title: "Beta" } },
+              },
+            ],
+          },
+        },
+        anyOf: [
+          { type: "object", title: "Outer A", properties: { x: {} } },
+          { type: "object", title: "Outer B", properties: { y: {} } },
+        ],
+      };
+      renderWithMantine(
+        <SchemaForm schema={nested} values={{}} onChange={vi.fn()} />,
+      );
+      const [outer, inner] = screen.getAllByRole("textbox", {
+        name: /Variant/,
+      });
+      await user.click(inner!);
+      await user.click(screen.getByRole("option", { name: "Inner B" }));
+      expect(screen.getByRole("textbox", { name: /Beta/ })).toBeTruthy();
+
+      await user.click(outer!);
+      await user.click(screen.getByRole("option", { name: "Outer B" }));
+      // The nested form is still mounted, so only a changed reset key can stop
+      // it displaying a branch the newly seeded values do not describe.
+      expect(screen.getByRole("textbox", { name: /Alpha/ })).toBeTruthy();
+    });
+
     it("renders no picker for a single-branch union but still shows its fields", () => {
       const schema: InspectorFormSchema = {
         type: "object",

clients/web/src/components/groups/SchemaForm/SchemaForm.tsx

@@ -972,8 +972,12 @@ export function SchemaForm({
                 handleFieldChange(fieldName, nestedValues)
               }
               disabled={disabled}
-              // Sub-fields belong to the same entity, so they reset with it.
-              resetKey={resetKey}
+              // Sub-fields belong to the same entity, so they reset with it —
+              // and to the branch it is being edited under, since two outer
+              // alternatives can both carry this field and a nested form left
+              // mounted across the switch would keep displaying the nested
+              // branch chosen for the other one.
+              resetKey={draftKey}
               // A nested form's invalid draft is the outer form's invalid draft,
               // so it reports through the same channel under this field's name.
               onValidityChange={(nestedInvalid) =>