Accept awaitable-returning async middleware callables

[sylvesterkaczmarek]

Summary

Accept function-style async middleware that is a synchronous callable returning an Awaitable, matching the SDK's public AsyncMiddlewareCallable type contract.

AsyncMiddlewareCallable is defined as a Callable[[APIRequest, AsyncCallNext], Awaitable[AsyncAPIResponse[Any]]]. That contract does not require the callable itself to be declared with async def: a regular function or callable object may validly return the coroutine produced by call_next(request).

validate_async_middleware() currently rejects that shape at client construction because it requires inspect.iscoroutinefunction(...) to be true.

Fix

For function-style async middleware, validate that the entry is callable but do not require the callable itself to be a coroutine function.

Class-based Middleware remains strict: handle_async() must still be async. Non-callable middleware remains rejected.

Regression coverage

Adds end-to-end async-client tests for:

• a regular function returning call_next(request);
• a callable object returning the awaitable;
• normal request completion through both wrappers;
• continued rejection of non-callable middleware.