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/tui/src/utils/schemaToForm.ts

@@ -481,6 +481,12 @@ export function missingRequiredFields(
         !admitsNull(property)
       );
     }
-    return value === undefined || value === "";
+    if (value === "") {
+      // A branch may pin its discriminator to the empty string, and the
+      // one-option control cannot produce anything else — reporting the seeded
+      // value as missing would make that branch permanently uncallable.
+      return constOf(properties[name]) !== "";
+    }
+    return value === undefined;
   });
 }