[Python AIO] Return EOF from UnaryStreamCall.read() as documented

[XuanWang-Amos]

Fix: #36581

Based on our documentation, we should return grpc.aio.EOF to indicate the end of the stream:

grpc/src/python/grpcio/grpc/aio/_base_call.py

Lines 166 to 178 in fb6a57b

	async def read(self) -> Union[EOFType, ResponseType]:
	"""Reads one message from the stream.
	Read operations must be serialized when called from multiple
	coroutines.
	Note that the iterator and read/write APIs may not be mixed on
	a single RPC.
	Returns:
	A response message, or an `grpc.aio.EOF` to indicate the end of the
	stream.
	"""

But if the call was intercepted, we're raising StopAsyncIteration, This Pr changes the return to match the documentation (Which is also the behavior for non-intercepted call).

[gnossen]

Looks great! Thanks for the fix

[gnossen]

I like the use of the subtest with the loop. It would be nice if we could also have a subtest that didn't include an interceptor at all. Running the exact same test against both an interceptor and a channel without an interceptor would help reinforce our principle that it should not be possible to tell whether a channel has an interceptor enabled or not externally.

I think you can change this by changing the iterable you're iterating over from Tuple[Callable[[], Interceptor]] to Tuple[List[Callable[[], Interceptor]]], and using [] to represent the variant with no interceptors.

[XuanWang-Amos]

Good idea, I'll add a subtest for channel without interceptor as well.

[gnossen]

CC @sourabhsinghs for visibility