Fix flakes in pauli_string_measurement_with_readout_mitigation_test

[pavoljuhas]

Ensure random Pauli strings have a non-identity operator for real.
Also add missing spaces to the exception message.

This fixes randomly-seed dependent test failure for

check/pytest -n0 --randomly-seed=3289494540 \
    cirq-core/cirq/contrib/paulistring/pauli_string_measurement_with_readout_mitigation_test.py::test_many_circuits_input_measurement_with_noise \
    cirq-core/cirq/contrib/paulistring/pauli_string_measurement_with_readout_mitigation_test.py::test_many_circuits_with_coefficient

cc: @ddddddanni

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.14%. Comparing base (e1deffc) to head (834eda1).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7185      +/-   ##
==========================================
- Coverage   98.14%   98.14%   -0.01%     
==========================================
  Files        1100     1100              
  Lines       96194    96191       -3     
==========================================
- Hits        94412    94408       -4     
- Misses       1782     1783       +1     

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[NoureldinYosri]

can this be a symptom of a real bug?

[pavoljuhas]

can this be a symptom of a real bug?

The test flakes were failing on Pauli string validation which rejects identity-only strings, for example, https://github.com/quantumlib/Cirq/actions/runs/14068636480/job/39397251223#step:7:402

The original test code intended to avoid such strings, but could in fact produce them with

operators = {q: random.choice(pauli_ops) for q in qubits}

This PR ensures that there is at least one non-identity Pauli in the operators dictionary, which prevents the flake.

[ddddddanni]

Thanks for fixing it! I recall adding the Pauli-I validation to the verification stage, but I overlooked the need to prevent the generation of all Pauli-I test strings in the test file. Thanks for catching the error and fix it quickly!