Enable classicEmberSupport() for compat-build scenarios

The min-supported, ember-lts-4.12, and ember-lts-5.12 try-scenarios all
set ENABLE_COMPAT_BUILD=true and install @embroider/compat + ember-auto-import
so they can exercise the classic Ember 4.x/5.12 resolution path. The compat
scenario was failing with:

    [vite]: Rollup failed to resolve import "ember-testing" from
    "test-app/tests/index.html?html-proxy&index=0.js".

`tests/index.html` has `<script type="module">import "ember-testing";</script>`
which is a bare specifier that needs the classic resolver. In the default
build, `ember()` is enough, but the compat path requires
`classicEmberSupport()` (see ef4/memory-scroll vite.config.mjs for the
reference pattern).

Spread `classicEmberSupport()` into plugins only when ENABLE_COMPAT_BUILD is
set, matching the pattern already used by the try scenarios themselves.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

Author: NullVoxPopuli

test-app/vite.config.mjs

@@ -1,9 +1,10 @@
 import { defineConfig } from 'vite';
-import { extensions, ember, hbs } from '@embroider/vite';
+import { extensions, ember, hbs, classicEmberSupport } from '@embroider/vite';
 import { babel } from '@rollup/plugin-babel';
 
 export default defineConfig({
   plugins: [
+    ...(process.env.ENABLE_COMPAT_BUILD ? [classicEmberSupport()] : []),
     hbs(),
     ember(),
     babel({