[FIX] web_editor: ensure dynamic placeholder focus

When inserting a dynamic placeholder in the editable document
we need to ensure the said document currently hold the focus and
have an active selection range.

This was previously handle by the `onDynamicPlaceholderClose()` method
which used to be called before `onDynamicPlaceholderValidate()`.

This seems to have changed from 16.1 to 16.2.

In order to fix the problem and ensure the focus in any situation,
we add a `wysiwyg.focus()` call at the beginning of the validate method.

task-3222474

closes odoo/odoo#124149

X-original-commit: 51b992dec0bcdf09fde5e0811699235b590723ee
Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/web_editor/static/src/js/backend/html_field.js

@@ -368,6 +368,9 @@ export class HtmlField extends Component {
     }
     onDynamicPlaceholderValidate(chain, defaultValue) {
         if (chain) {
+            // Ensure the focus is in the editable document
+            // before inserting the <t> element.
+            this.wysiwyg.focus();
             let dynamicPlaceholder = "object." + chain.join('.');
             dynamicPlaceholder += defaultValue && defaultValue !== '' ? ` or '''${defaultValue}'''` : '';
             const t = document.createElement('T');