test: let async-for loop exhaust to cover both branches

Remove time.sleep(1) so the child exits immediately after writing, then
drop the early-break guard.  The loop now terminates by exhaustion rather
than break, which covers the previously-missing 532->537 branch and
restores 100 % coverage.

Author: Christian-Sidak

tests/client/test_stdio.py

@@ -512,11 +512,10 @@ async def test_stdio_client_invalid_utf8():
     valid_line = '{"jsonrpc":"2.0","id":1,"method":"ping"}'
     script = textwrap.dedent(
         f"""
-        import sys, time
+        import sys
         sys.stdout.buffer.write(b"\\xff\\xfe\\n")
         sys.stdout.buffer.write({valid_line!r}.encode() + b"\\n")
         sys.stdout.buffer.flush()
-        time.sleep(1)
         """
     )
 
@@ -531,8 +530,6 @@ async def test_stdio_client_invalid_utf8():
         async with stdio_client(server_params) as (read_stream, _write_stream):
             async for item in read_stream:
                 items.append(item)
-                if len(items) >= 2:
-                    break
 
     assert len(items) == 2
     assert isinstance(items[0], Exception)