[php-src] Issue #23326: PHP 8.6 | Undocumented change in DOMDocument::$documentURI value format on Windows

Issue: https://github.com/php/php-src/issues/23326
Author: jrfnl

### Description

The following code:

```php
<?php
// Where $doc is a DOMNode instance.
$fileName  = $doc->ownerDocument->documentURI;
```

Resulted in this output on PHP 8.6-beta1:
```
D:\path\to\PHP_CodeSniffer\tests\Core\Generators\Fixtures\StandardWithDocs\Docs\Content\DocumentationTitlePCREFallbackStandard.xml
```

But I expected this output instead:
```
file:/D:/path/to/PHP_CodeSniffer/tests/Core/Generators/Fixtures/StandardWithDocs/Docs/Content/DocumentationTitlePCREFallbackStandard.xml
```

... which has been the format for that property value since at least PHP 5.4.

It is unclear to me whether this is a deliberate change or an accidental side-effect of something.

If it is a deliberate change, please document the change in the `UPGRADING` file.
If it is an accidental side-effect/unintentional change, please mark this as a bug in PHP 8.6.


### PHP Version

```plain
PHP 8.6.0beta1 (cli) (built: Aug 11 2026 17:58:37) (ZTS Visual C++ 2026 x64)
Copyright © The PHP Group and Contributors
Built by The PHP Group
Zend Engine v4.6.0-dev, Copyright © Zend by Perforce
    with Zend OPcache v8.6.0beta1, Copyright ©, by Zend by Perforce
```

### Operating System

Windows 10