[CodeQuality] Skip union types in AssertEqualsToSameRector to preserve loose type comparison

[TomasVotruba]

assertEquals() compares loosely, so it accepts values of different types across a union. Converting to strict assertSame() would break such tests.

Now skip when the actual (second) argument resolves to a union type.

#[DataProvider('stringProvider')]
public function testNormalizeStringValue(string|int|bool|\DateTime $input, string|int|bool|\DateTime $expected): void
{
    // union return type - values may differ in type between runs
    $this->assertEquals($expected, $this->formatterHelper->normalizeStringValue($input));
}

-$this->assertEquals($expected, $this->normalizeStringValue($input));
+$this->assertEquals($expected, $this->normalizeStringValue($input)); // unchanged

Before this change the rule wrongly narrowed it to assertSame().