Override ApiException.Message getter to include ResponseStatusCode

[koen-lee]

See #4349; the default exception details are not very helpful.
This PR intends to add the ResponseStatusCode, property of the base ApiException, to the message when no primary message extension is used.

It's not great, the suggestion from the above issue to add the description from the spec is better, but this is (hopefully) an incremental improvement, relatively low impact, and may serve as an example for future improvements by pointing at the relevant code.

Considerations:

• it interpolates a new string for every call, but there's no easy way around that since ResponseStatusCode is mutable.
• this could also be implemented in ApiException, but I think that would have higher impact and is less future proof: it's not in the path of fixing Take exception message from OpenAPI document #4349.

[baywet]

Thanks for the contribution!

We'll need to coordinate this change across languages to provide a consistent experience. Can you please update the other languages?

[koen-lee]

@baywet I'm only qualified to write/judge C# code, but if it's OK to have Claude and Copilot to write that code and you trust the unit tests and your own due process, I'll include other languages.

[koen-lee]

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

[baywet]

just for context I'm holding review until we merge #6953 since I believe those PRs will conflict a lot with each other.
Let me know if you have any additional comments or questions.

[koen-lee]

@baywet #6953 and this are independent - at least they are now that I have removed the errorcode key from the description in that PR.