Make reflection step work with various openrouter models

[Philipp-Userlike]

Why are these changes needed?

This is an attempt to address #5621 and general issues with OpenAI compatible providers and the reflection step.
I had the issue reproduced with both claude and gemini openrouter models and confirmed they can use the reflection step now.
And I think using tool_choice=none is a good idea in general

Related issue number

#5621

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[ekzhu]

Can you confirm this works for OpenAI endpoint? I hope we can add a general argument tool_choice to the base class methods so all model clients can support it.

[ekzhu]

@Philipp-Userlike see #6696

[Philipp-Userlike]

@ekzhu yes it also works with openai. I'm still clearing with my employer that I can agree to the policy, but I assume it will be fine.
The tool_choice general argument sounds good, I can adapt my PR to use that after it is done 👍

[Philipp-Userlike]

@microsoft-github-policy-service agree [company="Userlike"]

[Philipp-Userlike]

@microsoft-github-policy-service agree company="Userlike"

[ekzhu]

See #6697 has already been merged. @Philipp-Userlike do you want to create a new PR?