test: add dynamic macOS version input for DMG build workflow

.github/workflows/build-macos15-arm64-dmg.yml

@@ -3,6 +3,14 @@ name: Build macOS 15 DMG (arm64)
 on:
   workflow_dispatch:
     inputs:
+      macos_version:
+        description: macOS runner version
+        required: true
+        type: choice
+        options:
+          - "macos-15"
+          - "macos-26"
+        default: "macos-15"
       ref:
         description: Git ref to build (branch, tag, or sha)
         required: false
@@ -14,7 +22,7 @@ env:
 
 jobs:
   build-macos-dmg-arm64:
-    runs-on: macos-15
+    runs-on: ${{ inputs.macos_version }}
     steps:
       - name: Checkout
         uses: actions/checkout@v4