Replace deprecated asyncio.iscoroutinefunction usage in async_requests.py

[askpatrickw]

Running the latest Azure SDK stack on Python 3.14 emits deprecation warnings from msrest/universal_http/async_requests.py when processing async Bot Framework traffic:

DeprecationWarning: 'asyncio.iscoroutinefunction' is deprecated
and slated for removal in Python 3.16; use inspect.iscoroutinefunction() instead
  future = loop.run_in_executor(...)

asyncio.iscoroutinefunction was deprecated in Python 3.12 and will disappear in 3.16, so msrest should switch to inspect.iscoroutinefunction (or asyncio.coroutines.iscoroutinefunction if backward compatibility is required). This warning gets triggered every time a Teams AI bot posts or retrieves messages, making logs noisy and risking future runtime failures once the API is removed.

Please update msrest/universal_http/async_requests.py (and any similar call sites) to use inspect.iscoroutinefunction so the package stays compatible with upcoming Python versions.