Fix error and add test for mutable pauli string (quantumlib#5213)

quantumlib#4001 looks abandoned so this does the change and adds the test as requested.

cirq-core/cirq/ops/pauli_string.py

@@ -1546,6 +1546,6 @@ def _pauli_like_to_pauli_int(key: Any, pauli_gate_like: PAULI_GATE_LIKE):
             f'Expected {key!r}: {pauli_gate_like!r} to have a '
             f'cirq.PAULI_GATE_LIKE value. '
             f"But the value isn't in "
-            f"{list(PAULI_GATE_LIKE_TO_INDEX_MAP.values())!r}"
+            f"{set(PAULI_GATE_LIKE_TO_INDEX_MAP.keys())!r}"
         )
     return cast(int, pauli_int)

cirq-core/cirq/ops/pauli_string_test.py

@@ -1873,6 +1873,22 @@ def test_mutable_pauli_string_dict_functionality():
     assert b not in p
 
 
+@pytest.mark.parametrize(
+    'pauli', (cirq.X, cirq.Y, cirq.Z, cirq.I, "I", "X", "Y", "Z", "i", "x", "y", "z", 0, 1, 2, 3)
+)
+def test_mutable_pauli_string_dict_pauli_like(pauli):
+    p = cirq.MutablePauliString()
+    # Check that is successfully converts.
+    p[0] = pauli
+
+
+def test_mutable_pauli_string_dict_pauli_like_not_pauli_like():
+    p = cirq.MutablePauliString()
+    # Check error string includes terms like "X" in error message.
+    with pytest.raises(TypeError, match="PAULI_GATE_LIKE.*X"):
+        p[0] = 1.2
+
+
 def test_mutable_pauli_string_text():
     p = cirq.MutablePauliString(cirq.X(cirq.LineQubit(0)) * cirq.Y(cirq.LineQubit(1)))
     assert str(cirq.MutablePauliString()) == "mutable I"