Fix CI DMG packaging layout generation

[GcsSloop]

Summary

• replace transient bundle_dmg dependency with repo-managed appdmg generation
• keep the existing DMG background image and layout coordinates in a deterministic JSON spec
• extend packaging tests so CI catches fallback-to-plain-DMG regressions

Verification

• bash scripts/test/notarize_macos_test.sh
• bash scripts/test/package_local_release_test.sh
• TARGET_DIR="/Users/gcssloop/WorkSpace/AIGC/codex-router/desktop/src-tauri/target/universal-apple-darwin" APPLE_SIGNING_IDENTITY='' APPLE_API_KEY_PATH='' APPLE_API_KEY_ID='' APPLE_API_ISSUER='' bash scripts/desktop/notarize_macos.sh