Closes #10227 RelayState max length not respected

@ahus1: Can you approve running the workflows?

Hi @evtr, thanks for this PR.

Two things I note here:

• Please put the Closes #10227 in the very first comment of the issue. Only then will it link the PR to the issue.
• This project doesn't use Mockito. If you can't write the test without it, there needs to be an integration test for this new feature. One of those SAML integration tests is for example KcSamlXPathAttributeMapperTest.

Please let me know if you can think of a way to write such a test without Mockito, or if you need help writing such a SAML test case.

The objects that are mocked in the tests mocks very extensive interfaces. I can create TestClientModel and TestRealmModel classes but there are many methods to override. Is that ok?

@ahus1: I changed the unit tests to not using mockito

@ahus1: I don't really understand what is failing here? Can you help?

Hi @evtr - failing Quickstart tests are unrelated to your changes. I'll figure out what needs to be done there.

I'll also reach out to some maintainers in a separate channel to find out whether to go with the manually created mocks, or with a SAML integration tests.

My thoughts one the SAML integration tests. Perhaps I shouldn't create new tests but choose one of the "sunshine" scenarios that are already in the test suite and add an assertion on the relaystate length somehow?

@hmlnarik - this PR adds a class IdentityBrokerStateTestHelpers.java to avoid using a mocking framework like Mockito as I know that adding such dependencies is discouraged.

Could you please provide a hint if this is the way to go, or if the contributor should go for an Arquillian based integration test.

Thanks!