fix: validate decision nodes have at least 2 children in frontend and AI builder

The frontend validate button was not checking that decision nodes with
children have at least 2 branches, so it would pass validation but the
backend publish check would reject with a 422. The AI tree validator also
only checked options count, not children count — so AI-generated trees
with 2 options pointing to the same single child would pass generation
validation but fail at publish time.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/store/treeEditorStore.ts

@@ -710,7 +710,17 @@ export const useTreeEditorStore = create<TreeEditorState>()(
                 message: `Decision node "${node.id}" requires at least one option`,
                 severity: 'error'
               })
-            } else {
+            }
+            // Decision nodes with children must have at least 2 branches
+            if (node.children && node.children.length > 0 && node.children.length < 2) {
+              errors.push({
+                nodeId: node.id,
+                field: 'children',
+                message: `Decision node "${node.id}" must have at least 2 branches`,
+                severity: 'error'
+              })
+            }
+            if (node.options && node.options.length > 0) {
               // Validate options
               node.options.forEach((opt, i) => {
                 if (!opt.label?.trim()) {