[Php81] Skip readonly on property returned by reference in ReadOnlyPropertyRector

[TomasVotruba]

Problem

ReadOnlyPropertyRector made a promoted property readonly even when it is returned by reference from a & method(). Returning by reference lets callers hold and mutate the value, so the property cannot be readonly — PHP throws a fatal "Cannot acquire reference to readonly property" at runtime.

Spotted in the wild in infection: https://github.com/infection/infection/blob/HEAD/src/TestFramework/Tracing/Trace/TestLocations.php

final class TestLocations
{
    public function __construct(
        private array $byLine = [],
        private readonly array $byMethod = [],
    ) {
    }

    public function &getTestsLocationsBySourceLine(): array
    {
        return $this->byLine;
    }
}

The rule rewrote private array $byLine to private readonly array $byLine, which breaks the by-reference getter.

Fix

Add isPropertyReturnedByRef() guard. Before making a property/promoted param readonly, bail out if any method declared with & (returns by reference) returns $this-><property> (directly or as the base of a nested expression).

Added skip_returned_by_ref.php.inc fixture covering the case.