Remove #[\ReturnTypeWillChange] in favor of proper return types

[Copilot]

PHP 8.4 deprecated implicitly nullable parameter types, which was fixed in #43. This PR completes the cleanup by replacing #[\ReturnTypeWillChange] suppression attributes with actual return type declarations, now that minimum PHP is 8.0.

• jsonSerialize(): mixed — matches JsonSerializable interface
• offsetSet(): void — matches ArrayAccess interface
• offsetUnset(): void — matches ArrayAccess interface

// Before: suppress the deprecation
#[\ReturnTypeWillChange]
public function jsonSerialize() { ... }

// After: declare the type
public function jsonSerialize(): mixed { ... }

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This change is 

[coveralls]

coverage: 97.987%. remained the same — copilot/fix-php-compatibility-warnings into master