fix(shared): merge $defs instead of replacing in materializeDynamicRefBinding

When materializing a dynamic ref binding, the caller's $defs now merge
with the template's $defs instead of wholesale replacing them. This
preserves helper sub-schemas that the template may reference from its
body alongside the anchor placeholders.

Author: aqeelat

packages/shared/src/openApi/3.1.x/parser/__tests__/dynamicRef.test.ts

@@ -430,6 +430,38 @@ describe('materializeDynamicRefBinding', () => {
 
     expect(result!.description).toBe('overridden');
   });
+
+  it('merges $defs from refSchema and caller schema', () => {
+    mockResolveRef.mockReturnValue({
+      $defs: {
+        helper: { type: 'string' },
+        placeholder: { $dynamicAnchor: 'itemType', not: {} },
+      },
+      type: 'object',
+    });
+
+    const result = materializeDynamicRefBinding({
+      context: createContext(),
+      schema: {
+        $defs: {
+          itemType: {
+            $dynamicAnchor: 'itemType',
+            $ref: '#/components/schemas/User',
+          },
+        },
+        $ref: '#/components/schemas/Template',
+      },
+    });
+
+    expect(result!.$defs).toEqual({
+      helper: { type: 'string' },
+      itemType: {
+        $dynamicAnchor: 'itemType',
+        $ref: '#/components/schemas/User',
+      },
+      placeholder: { $dynamicAnchor: 'itemType', not: {} },
+    });
+  });
 });
 
 describe('shouldInlineDynamicRefTarget', () => {

packages/shared/src/openApi/3.1.x/parser/dynamicRef.ts

@@ -191,6 +191,12 @@ export function materializeDynamicRefBinding({
     ...refSchema,
     ...schema,
   };
+  if (refSchema.$defs && schema.$defs) {
+    materializedSchema.$defs = {
+      ...refSchema.$defs,
+      ...schema.$defs,
+    };
+  }
   delete (materializedSchema as Record<string, unknown>).$ref;
   delete (materializedSchema as Record<string, unknown>).$dynamicAnchor;
   delete (materializedSchema as Record<string, unknown>).$id;