fix: validate http_client type in AsyncAPIClient

[EaCognitive]

Fixes #1095

Problem

When a user passes a synchronous httpx.Client to AsyncOpenAI via the http_client parameter, the library crashes during a request with a cryptic TypeError: object Response can't be used in 'await' expression. This happens because the SDK expects an asynchronous client and tries to await the result of .send(), which in a sync client returns a Response object instead of a coroutine.

Solution

Added a type validation check in AsyncAPIClient.__init__. If an http_client is provided, we now explicitly verify that it is an instance of httpx.AsyncClient. If not, we raise a clear TypeError during initialization.

Changes

• src/openai/_base_client.py: Added isinstance check for http_client in AsyncAPIClient.__init__.

Verification

• Verified that passing a sync client now raises a helpful TypeError instead of crashing later with a cryptic one.