Introduce foundation for unit tests (pytest + Zuul CI)

[berendt]

Set up the minimal infrastructure so unit tests can be added in follow-up issues. Covers #2192.

• Add [dev-packages] in Pipfile: pytest, pytest-cov, pytest-mock
• Create tests/ layout with a smoke test asserting osism is importable and exposes version
• Configure pytest via [tool:pytest] in setup.cfg
• Add playbooks/test-unit.yml and a new python-osism-unit-tests Zuul job wired into the check and periodic-daily pipelines; the existing python-osism-test-setup job is kept in parallel (replace decision left to review per the issue)
• Document local test execution in README.md

Closes #2192

AI-assisted: Claude Code

[sourcery-ai]

Hey - I've found 1 issue

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="tests/unit/test_smoke.py" line_range="10-11" />
<code_context>
+    assert osism is not None
+
+
+def test_package_has_version_attribute():
+    assert hasattr(osism, "__version__")
</code_context>
<issue_to_address>
**suggestion (testing):** Extend the version smoke test to validate the value of `__version__`, not only its presence.

`hasattr(osism, "__version__")` would still pass if `__version__` were `None` or an unexpected type. To make the test more robust, also assert that it is a non-empty string, for example:

```python
assert isinstance(osism.__version__, str)
assert osism.__version__
```
</issue_to_address>

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