fix: reuse the same page for every call forwarded by On

[tiagoabsantos]

What:

• Bug Fix
• New Feature

Description:

On::__call() created a new PendingAwaitablePage on every call, so $page = visit('/')->on()->mobile() opened a new browser context for each separate $page->... call. A click() changed one page and the next assertion ran on a fresh one. On now creates the pending page once and forwards every call to it, like PendingAwaitablePage does with its webpage.

$page = visit('/')->on()->mobile();

$page->click('#toggle');
$page->assertSeeIn('#result', 'Clicked');

tests/Browser/Visit/OnTest.php covers separate calls with and without a device.

Related:

Fixes pestphp/pest#1931

[DaniilSkLi]

I hit this bug (pestphp/pest#1931) and opened a PR with a slightly
different approach: #274

Two differences worth discussing:

1. I kept the class readonly — the lazy ??= initialization of an
   uninitialized (implicitly readonly) property is valid at runtime
   (initialized once from inside class scope), it just needs two targeted
   @phpstan-ignore comments, same as the existing @phpstan-ignore-next-line
   already used in that method.
2. I added the missing @mixin Webpage|AwaitableWebpage to On, otherwise IDE
   autocompletion for ->on()->...() stays broken due to recursive @mixin
   resolution.

[tiagoabsantos]

On follows PendingAwaitablePage: nullable property, ??=, class not readonly. A one-time write to an uninitialized readonly property is valid PHP. PHPStan still reports property.uninitializedReadonly and property.readOnlyAssignNotInConstructor, so I left the class mutable like PendingAwaitablePage.

The mixin point is right. IDEs that don't follow @mixin PendingAwaitablePage never see Webpage or AwaitableWebpage, so on()->click() has no completion. From has the same annotation. This PR is only the extra browser context on each call. I'll add @mixin Webpage|AwaitableWebpage on On if you want that in here too.

[DaniilSkLi]

Up to you on scope. If you'd rather keep this PR focused on the context bug, that's fine — the @mixin fix for On is already in #274. Either way works for me.

Side note: while checking this I noticed From has the same @mixin, but it has no __call - the annotation advertises methods that would throw at runtime (Call to undefined method). City methods return PendingAwaitablePage directly, so the mixin is just misleading there and could be dropped in a follow-up.