DataView: add notices config to suppress built-in save notices

[zinigor]

Problem

When a DataView page lives under the WordPress Settings menu, two "Settings saved." notices appear after every save:

1. WordPress core — for pages under options-general.php, admin-header.php includes wp-admin/options-head.php, whose back-compat block fires for ?page=...&updated=1:

   if ( isset( $_GET['updated'] ) && isset( $_GET['page'] ) ) {
       // For back-compat with plugins that don't use the Settings API and just set updated=1 in the redirect.
       add_settings_error( 'general', 'settings_updated', __( 'Settings saved.' ), 'success' );
   }

   This is the native notice with id="setting-error-settings_updated". DataView's redirect (updated=1) is exactly the case this path was written for.

2. DataView — RequestRouter::render_notices() also fires on isset($_GET['updated']), echoing a second notice in the page body.

Both key on the same ?updated param, so a consumer can't suppress just one.

Change

Add a notices config option (bool, default true). When false, render_notices() returns early, letting the host environment own save notices:

new DataView([
    'slug'    => 'my_settings',
    'mode'    => 'singular',
    'storage' => 'option',
    'notices' => false, // host (WP Settings) renders the save notice
    'fields'  => [ /* ... */ ],
]);

Validation/error output (render_errors()) is a separate path and is unaffected.

Tests

4 new tests: config default (true), config override (false), and render_notices() output present-when-enabled / empty-when-disabled. Full suite: 274 passing, 1 pre-existing unrelated skip.

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