Use inspect.isawaitable in AsyncClientResiliencyInterceptor

Replace the ad-hoc `hasattr(result, '__await__')` check in
`AsyncClientResiliencyInterceptor._record_outcome` with the canonical
`inspect.isawaitable` predicate, and tighten the `on_recreate` callback
annotation to `Callable[[], Union[None, Awaitable[object]]]` so it reflects
the actual contract (sync callbacks return None, async callbacks return an
Awaitable that we await).

Addresses the github-code-quality 'Statement has no effect' warning surfaced
on PR #135 by making the awaitable check explicit and type-driven.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Bernd Verst

durabletask/internal/grpc_resiliency.py

@@ -1,10 +1,11 @@
 # Copyright (c) Microsoft Corporation.
 # Licensed under the MIT License.
 
+import inspect
 import random
 import threading
 from dataclasses import dataclass, field
-from typing import Callable, Optional
+from typing import Awaitable, Callable, Optional, Union
 
 import grpc
 import grpc.aio
@@ -125,7 +126,7 @@ class AsyncClientResiliencyInterceptor(grpc.aio.UnaryUnaryClientInterceptor):
     def __init__(
             self,
             failure_tracker: FailureTracker,
-            on_recreate: Callable[[], "Optional[object]"],
+            on_recreate: Callable[[], Union[None, Awaitable[object]]],
     ):
         self._failure_tracker = failure_tracker
         self._on_recreate = on_recreate
@@ -147,7 +148,7 @@ async def _record_outcome(self, method: str, error: Optional[BaseException]) ->
         if status_code is not None and is_client_transport_failure(method, status_code):
             if self._failure_tracker.record_failure():
                 result = self._on_recreate()
-                if hasattr(result, "__await__"):
+                if inspect.isawaitable(result):
                     await result
         else:
             self._failure_tracker.record_success()