Fix scalar message header conversion to nullable enums

[lifinsky]

Summary

Fix scalar message header conversion for nullable enum handler parameters.

Problem

When a message handler declares a nullable enum header parameter:

#[Header('deliveryMethod')]
?DeliveryMethod $deliveryMethod = null

Ecotone represents the target type as the DeliveryMethod|null union.

ScalarToEnumConverter correctly recognizes the target as an enum-compatible type, but then passes the complete union type string to ReflectionEnum. This results in the following error for a non-null scalar header value:

Class "DeliveryMethod|null" does not exist

A missing header or a header with a null value is not affected because scalar-to-enum conversion is not required in that case.

Fix

When the target is a union type, remove null before creating ReflectionEnum and converting the scalar value to the enum case.

Commit Structure

1. Add a regression test that reproduces the failure without the fix.
2. Add the minimal fix to ScalarToEnumConverter.

Verification

Before the fix, the regression test fails with:

Class "DeliveryMethod|null" does not exist

After the fix:

OK (1 test, 1 assertion)

The existing Core test suite also passes.

Pull Request Contribution Terms

• I have read and agree to the contribution terms outlined in CONTRIBUTING.

[lifinsky]

@dgafka Hi, I need your help :)

[dgafka]

Make sense, thanks for the fix :)