Improve error messaging for AI Assistant when WordPress.com connection is missing

[Copilot]

Users without WordPress.com connection attempting to use AI Assistant features (featured image generation, content completion) received the generic WordPress error "Sorry, you are not allowed to do that." This provided no actionable information about the connection requirement.

Changes

Modified Jetpack_AI_Helper to intercept connection-related errors and return clear, actionable messages:

• get_dalle_generation(): Detects unavailable_site_id and missing_token errors, returns: "Your account must be connected to WordPress.com to generate AI images. Please connect your account from the Jetpack settings screen to proceed."

• get_gpt_completion(): Same error detection pattern with contextually appropriate messages for text generation.

if ( is_wp_error( $site_id ) ) {
    if ( 'unavailable_site_id' === $site_id->get_error_code() ) {
        return new WP_Error(
            'unavailable_site_id',
            __( 'Your account must be connected to WordPress.com to generate AI images. Please connect your account from the Jetpack settings screen to proceed.', 'jetpack' ),
            array( 'status' => 403 )
        );
    }
    return $site_id;
}

Error codes and HTTP status codes remain unchanged for backward compatibility.

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