fix: fix batch processor

The change to BasePartialProcessor.async_process() in commit d08711f
causes the method to create a new loop on every invocation. This is
because asyncio.get_running_loop() raises RuntimeError also when the
thread has an event loop, since the loop is no longer running after
loop.run_until_complete() terminates.

Author: exg

aws_lambda_powertools/utilities/batch/base.py

@@ -135,7 +135,7 @@ async def async_process_closure():
             # Python 3.14+ will raise RuntimeError if get_event_loop() is called when there's no running loop
             # We need to handle both cases: existing loop (container reuse) and no loop (cold start)
             try:
-                loop = asyncio.get_running_loop()
+                loop = asyncio.get_event_loop()
             except RuntimeError:
                 # No running loop, create a new one
                 loop = asyncio.new_event_loop()