Feat/improved prompt handling

[equist]

Description

Improvments of prompt and max_age parameter handling.

Type of change

[ ] Bug fix
[ x ] Feature
[ ] Refactoring
[ ] Documentation
[ ] Other

Implementation of feature #54.

Does this PR introduce a breaking change?

[ x ] Yes
[ ] No

The old code ignored unsupported prompt modes, but this implementation returns an error to the client.

Testing

I have fixed an error in an existing integration test.
There are several new integration tests.

LLM Usage

I have not used LLM, except for the extended auto-complete feature primarly used for XML comments.
I used co-pilot to set up some structure for the IdentityServerApplicationBuilderExtensionsTests class.

Other context

I believe this implementation is better aligned with current Duende IdentityServer implementation, but it is a breaking change compared to IdSrv 4 code base which ignored unsupported prompt values.

[equist]

I hope that I have fixed all your comments now. I'm sorry for missing unit tests and I'll do better next time :)

[JoStevensRSK]

Can we remove this PromptProcessed check.
The whole request is meant to be replayed and validated once authentication has taken place. If the login took more time than the max age then it should be failed. It also complicates the flow and requires additional constant values.