Replace `#[\ReturnTypeWillChange]` with native return type declarations

[WarLikeLaux]

The framework currently uses #[\ReturnTypeWillChange] in 17 files (72 methods) to suppress PHP 8.1+ deprecation warnings. This attribute will stop working in PHP 9.0, causing fatal errors for any method with an incompatible return type.

• https://wiki.php.net/rfc/internal_method_return_types
• https://php.watch/versions/8.1/ReturnTypeWillChange
• https://www.php.net/manual/en/class.returntypewillchange.php

In PHP 9.0, incompatible return types in methods overriding internal methods will result in a fatal error, regardless of whether #[\ReturnTypeWillChange] is present.

Affected files

File	Methods	Risk
web/SessionIterator.php	5	Low (internal class)
web/SessionHandler.php	5	Low (internal class)
web/HeaderCollection.php	5	Low
web/CookieCollection.php	5	Low
web/Session.php	5	Medium
db/BatchQueryResult.php	5	Low (internal class)
db/DataReader.php	5	Low
db/JsonExpression.php	1	Low
db/ArrayExpression.php	5	Low
db/SqlToken.php	5	Low
db/mssql/DBLibPDO.php	2	Low (internal class)
db/mssql/PDO.php	2	Low (internal class)
db/mssql/SqlsrvPDO.php	2	Low (internal class)
db/BaseActiveRecord.php	5	High (widely extended)
base/Model.php	5	High (widely extended)
base/ArrayAccessTrait.php	5	High (used by Model)
caching/Cache.php	5	Medium

BC impact

Adding return types to non-final classes is technically a minor BC break: any child class overriding these methods without matching return types will get a fatal error. However, most of these methods implement PHP built-in interfaces (Iterator, ArrayAccess, Countable) and child classes will need the same return types anyway for PHP 9.0 compatibility.