test: add start() not-entered guard to completeness test

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

Author: Varun Sharma

tests/shared/test_task_group.py

@@ -145,6 +145,8 @@ async def test_task_group_not_entered_raises() -> None:
         ctg.cancel_scope
     with pytest.raises(RuntimeError, match="not been entered"):
         ctg.start_soon(lambda: None)
+    with pytest.raises(RuntimeError, match="not been entered"):
+        await ctg.start(lambda: None)
 
 
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