Quote python-version value in the tests workflow

[albertvillanova]

This PR quotes the python-version value in the tests workflow, making it consistent with every other setup-python usage in the repository.

Follow-up to #6881.

Motivation

In YAML, an unquoted 3.12 is parsed as a float rather than a string. It happens to work here, but it is the classic pitfall behind versions such as 3.10 silently becoming 3.1. Every other setup-python usage in the repository already quotes the value.

Changes

• Quote the python-version value in the code quality job of the tests workflow, the only remaining unquoted occurrence

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Note

Low Risk
One-line CI YAML quoting change with no runtime or security impact.

Overview
Quotes python-version as '3.12' in the check_code_quality job of tests.yml.

This matches other setup-python usages and avoids YAML parsing the version as a float.

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[albertvillanova]

Merging this PR as trivial change.