From syntax highlight

The contact form can contain html code.
I suggest to add this code into ContactForm7:

add_action(
    'admin_enqueue_scripts',
    function (string $hook): void {
        if ($hook === 'toplevel_page_wpcf7' && isset($_GET['post'])) {
            $type = 'text/html';
            $settings = wp_enqueue_code_editor(
                [
                    'type' => $type,
                    'htmlhint' => ['space-tab-mixed-disabled' => 'space'],
                    'codemirror' => ['indentWithTabs' => false]
                ]
            );
            if ($settings !== false) {
                wp_add_inline_script(
                    'code-editor',
                    sprintf(
                        <<<'EOF'
                        jQuery(function () {
                          const codemirror = wp.codeEditor.initialize('wpcf7-form', %s).codemirror
                          wpcf7.taggen.insert = content => {
                            let cursor = codemirror.getCursor()
                            codemirror.replaceRange(content, cursor)
                            cursor = codemirror.getCursor()
                            codemirror.focus()
                            codemirror.setCursor(cursor)
                          }
                        })
                        EOF,
                        wp_json_encode($settings)
                    )
                );
            }
        }
    }
);

This will activate CodeMirror code editor integrated in WordPress and highlight HTML code if it is used in a Contact Form.