Fix: Fail fast when multiple themes are active in a build (fixes #3784)

[swashbuck]

Fixes #3784

Fix

• Add a check-plugins grunt task that detects when more than one theme is active after accounting for build.excludes, build.includes, productionExcludes, and --theme CLI flag
• Chain check-plugins from check-json via grunt.task.run() so it runs for all entry points (direct grunt, rub-cli, server-build)
• Also register check-plugins in the build and dev task pipelines for explicit visibility

Note: Multiple menus are intentional (main menu + sub menu), so only themes are validated.

Testing

1. Install two themes (e.g. adapt-contrib-vanilla plus a custom theme)
2. Set _theme in config.json to the custom theme but do NOT add vanilla to build.excludes
3. Run grunt build or rub -cF - verify the build exits with a clear error naming the conflicting themes and suggesting the fix
4. Add the extra theme to build.excludes and re-run - verify the build proceeds normally
5. Run with --theme=adapt-contrib-vanilla flag - verify the check is skipped (CLI flag overrides)

[oliverfoster]

Multi-menu courses is intentional. Sometimes a main menu and a sub menu are different.

[swashbuck]

Multi-menu courses is intentional. Sometimes a main menu and a sub menu are different.

Updated to only apply when multiple themes are present.

Why themes and not menus?
Both <%= theme %> and <%= menu %> default to ** in the grunt config, so both compile all installed plugins of their type. Menus have very few LESS files (typically 3-4 across all installed menus), so the heap impact is negligible. Themes can have 80+ LESS files each with deep variable/mixin dependency chains, making two themes together enough to OOM the process.