fix(ci): install pytest for python sdk omnibus test

[stevo1403]

Summary

• install pytest in the Python SDK CI job
• run tests via python -m pytest for consistent invocation

Why

The Test Python SDK check fails in CI with pytest: command not found because packages/openmemory-py does not define a [dev] extra in pyproject.toml, so pip install -e .[dev] does not install pytest.

Validation

• Verified failure in run 22349964921 (Run Omnibus Test (Deep Verify) step).
• This PR updates .github/workflows/ci.yml only.

[Copilot]

Pull request overview

This PR fixes the Python SDK CI job by ensuring pytest and its dependencies are installed before running the test suite. The pyproject.toml for the Python SDK doesn't define a [project.optional-dependencies] section with a dev extra, so the pip install -e .[dev] command was not installing pytest, causing test failures.

Changes:

• Added explicit pip install pytest to ensure pytest is available for test execution
• Changed test invocation from pytest to python -m pytest for consistent Python environment usage

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