Remove accidental test skips from test_stdio.py

Remove pytest.mark.skip decorators that were accidentally added,
restoring the file to match main branch.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: pcarleton

tests/client/test_stdio.py

@@ -246,7 +246,6 @@ class TestChildProcessCleanup:
 
     @pytest.mark.anyio
     @pytest.mark.filterwarnings("ignore::ResourceWarning" if sys.platform == "win32" else "default")
-    @pytest.mark.skip(reason="Flaky test - timing-dependent process cleanup")
     async def test_basic_child_process_cleanup(self):
         """
         Test basic parent-child process cleanup.
@@ -341,7 +340,6 @@ async def test_basic_child_process_cleanup(self):
 
     @pytest.mark.anyio
     @pytest.mark.filterwarnings("ignore::ResourceWarning" if sys.platform == "win32" else "default")
-    @pytest.mark.skip(reason="Flaky test - timing-dependent process cleanup")
     async def test_nested_process_tree(self):
         """
         Test nested process tree cleanup (parent → child → grandchild).
@@ -440,7 +438,6 @@ async def test_nested_process_tree(self):
 
     @pytest.mark.anyio
     @pytest.mark.filterwarnings("ignore::ResourceWarning" if sys.platform == "win32" else "default")
-    @pytest.mark.skip(reason="Flaky test - timing-dependent process cleanup")
     async def test_early_parent_exit(self):
         """
         Test cleanup when parent exits during termination sequence.