Test against ember 6.12 LTS, and fix the release/beta/canary scenarios

[NullVoxPopuli-ai-agent]

Adds an ember-lts-6.12 scenario and fixes ember-release, ember-beta, and ember-canary. Those now run ember-source 7.3, 7.4, and 7.5, and they failed on main.

ember-source 7 is a v2 addon with development/production export conditions. The vite-app needed these changes to build and test against it:

• ember-cli 6.9 → 7.3. 6.9 reads classic vendor files from ember-source and crashes.
• Removed ember-cli-htmlbars, and bumped tracked-built-ins to 4.1.2 to drop its ember-tracked-storage-polyfill. Both brought in the classic ember-cli-htmlbars, which asks ember-source for a template compiler path that no longer exists.
• babel-plugin-ember-template-compilation 4, with no compilerPath, like the current app blueprint.
• New tests/vite-app/.env.development with NODE_ENV=development. vite build --mode development still sets NODE_ENV=production, so the test build got the production ember-source and assert from @ember/debug did nothing. modifier() must be invoked with a function failed for that reason.
• ember-try runs the test:ember script, not plain ember test.
• Bumped @embroider/*, @ember/test-helpers (to 5.5.1), and ember-qunit to the blueprint versions. 5.5.0 painted synchronously inside render() on ember 6.8+, which broke the getPromiseState "handles async function" test. 5.5.1 fixes that (Render via renderComponent from the render queue, like the legacy outlet path emberjs/ember-test-helpers#1576), so no test changes are needed.

Checked locally: ember try:one passes 103/103 for 5.12, 6.4, 6.8, 6.12, release, beta, and canary, and the default pnpm test:ember passes too. After the 5.5.1 bump: default, 5.12, 6.8, 6.12, release, beta, and canary pass 103/103. The test-app and min-supported apps did not change.

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