`ChunkReflow.verify()` fails on `test_from_auto_rewrite_xyz` and message missformat

[AlMrvn]

In stimflow/_chunk/_chunk_reflow_test.py, the existing test test_from_auto_rewrite_xyz tests creating an N-to-1
reflow chunk where Logical $X$ and $Z$ inputs combine into a $Y$ output:

def test_from_auto_rewrite_xyz():
    result = stimflow.ChunkReflow.from_auto_rewrite(
        inputs=[stimflow.PauliMap({"X": [2, 3]}), stimflow.PauliMap({"Z": [2, 3]})],
        out2in={stimflow.PauliMap({"Y": [2, 3]}): "auto"},
    )
    assert result == stimflow.ChunkReflow(
        out2in={
            stimflow.PauliMap({"Y": [2, 3]}): [stimflow.PauliMap({"X": [2, 3]}), stimflow.PauliMap({"Z": [2, 3]})]
        }
    )

If you add result.verify() to the end of test_from_auto_rewrite_xyz, it crashes with:

 ValueError: not enough values to unpack (expected 2, got 1)

The issue come ChunkReflow.verify() (around line 299), .verify() asserts:

if len(self.out2in) != len(self.removed_inputs):
    ...
    raise ValueError("\n".join(msg))

on top, there is a small bug, when len(self.out2in) != len(self.removed_inputs) fails and verify() attempts to format the exception message, lines 301 and 305 execute:

  for ps, obs in self.out2in:
      msg.append(f"    {ps}, obs={obs}")

it's missing a .items() and this lead to the Value Error instead of the actual helpful message about the size of the out2in and removed inputs