Fix duplicate Elementor promotion control registration

[code-snippets-bot]

Problem

Code Snippets 3.10.2 can register the same Elementor promotion control twice while loading the Elementor editor. Elementor then reports:

Function Elementor\Controls_Manager::add_control_to_stack was called incorrectly. Cannot redeclare control with same name "code_snippets_promotion_notice_elementor_pro".

The notice occurs with Code Snippets active even when all individual snippets are disabled, and disappears when Code Snippets is deactivated.

Root cause

Plugin::load_plugin() instantiates Promotion_Manager twice on admin requests:

if ( is_admin() ) {
    $this->admin = new Bootstrap_Admin();
    new Promotion_Manager();
}

// ...

if ( is_admin() ) {
    new Promotion_Manager();
}

Each Promotion_Manager creates an Elementor_Editor, which registers another elementor/init callback. Both instances then attach callbacks that add the same code_snippets_promotion_notice_elementor_pro control.

The duplicate initialization is present in the 3.10.2 release and remains present on core and core-beta.

Expected behavior

• Promotion_Manager is initialized once per request.
• The Elementor Core and Elementor Pro promotion controls are each registered once.
• Opening the Elementor editor does not produce a duplicate-control notice.

Acceptance criteria

• Remove the duplicate Promotion_Manager initialization.
• Verify Elementor and Elementor Pro editor loading with Code Snippets active.
• Add regression coverage that prevents the promotion integration from being initialized twice.