refactor!: improve types in platform api

[chr-hertel]

I was just tired of doing those assert($response instanceof ...); statements
also resolves the special status of AsyncResponse by decoupling it from the ResponseInterface

WDYT?

[smnandre]

I like this very much... it does have its own drawbacks, but I feels it's by far the best way out of this situation, and it clarifies a lot of things on a DX level

[DZunke]

This looks pretty cool! Thanks @chr-hertel - those assertions you mentioned are all over the place and this looks like a cool solution. Mostly one knows what kind of request was sent and what should be the response but for type safety checks where required. Wrapping those checks to the library with the ResponsePromise feels like a good step forward!

[chr-hertel]

This PR demonstrates the benefits for lib users quite well I guess:
https://github.com/php-llm/llm-chain-symfony-demo/pull/29/files