Remove deprecated asyncio.iscoroutinefunction fallback (#3767)

The `asyncio.iscoroutinefunction` call is deprecated in Python 3.14
and slated for removal in 3.16. Since `is_coroutine_function` already
unwraps `functools.partial` layers before checking, the asyncio
fallback is redundant on all supported Python versions — it can never
return True when `inspect.iscoroutinefunction` returned False on the
unwrapped function.

Fixes #3765

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/fastmcp/utilities/async_utils.py

@@ -1,6 +1,5 @@
 """Async utilities for FastMCP."""
 
-import asyncio
 import functools
 import inspect
 from collections.abc import Awaitable, Callable
@@ -21,7 +20,7 @@ def is_coroutine_function(fn: Any) -> bool:
     """
     while isinstance(fn, functools.partial):
         fn = fn.func
-    return inspect.iscoroutinefunction(fn) or asyncio.iscoroutinefunction(fn)
+    return inspect.iscoroutinefunction(fn)
 
 
 async def call_sync_fn_in_threadpool(