fix: await response.code() and use sys.exc_info() in async gRPC interceptor

[danishashko]

Fixes #1059

Two bugs in _AsyncExceptionInterceptor._handle_grpc_failure_async():

Bug 1: response.exception() raises AttributeError

grpc.aio.Call objects don't have an .exception() method. That method only exists on synchronous grpc.Future objects. When a timeout or temporary service error triggers the async interceptor, calling response.exception() raises AttributeError: 'UnaryUnaryCall' object has no attribute 'exception', masking the original gRPC error entirely.

Fix: use sys.exc_info()[1] to capture the live exception — this works since _handle_grpc_failure_async is always called from within an except grpc.RpcError block in the async wrappers.

Bug 2: response.code() not awaited

In grpc.aio, Call.code() is a coroutine and must be awaited. Without await, status_code would be a coroutine object, making the status_code not in _RETRY_STATUS_CODES check always evaluate to True, meaning retry-able errors (INTERNAL, RESOURCE_EXHAUSTED) would incorrectly be parsed as GoogleAdsFailures instead of being re-raised as-is.

Other cleanup:

• Removed leftover reasoning comments that were accidentally committed
• Import _RETRY_STATUS_CODES from interceptor.py instead of redefining it locally

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[ZLeventer]

Good fix. Two real bugs here:

1. await response.code() — grpc.aio.Call.code() is a coroutine, unlike the sync grpc.Future.code() which returns immediately. Calling it without await would return a coroutine object, not the status code, causing the not in _RETRY_STATUS_CODES check to always be True (a coroutine is never equal to a status code enum).

2. sys.exc_info()[1] instead of response.exception() — grpc.aio.Call doesn't expose .exception() the same way sync grpc.Future does. Using sys.exc_info() within the except block is the correct approach since this method is always called from an exception handler.

Also good cleanup: removing the commented-out reasoning/notes and the unreachable raise response.exception() at the bottom. The import of _RETRY_STATUS_CODES from the module instead of redefining it locally is the right call — DRY and ensures any future changes to the retry set apply everywhere.