🐛 Complete QDMI child-device string conversions

[burgholzer]

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Description

Complete the public qdmi::toString mappings for the child-device session parameter and device property introduced with child-device support. This keeps diagnostics and property-name formatting exhaustive for the current QDMI enum surface.

This small fix was identified while reviewing #1912 and is extracted because it has no dependency on configurable device discovery. The existing child-device changelog entry now also attributes this follow-up to #1952 and retains the original human author attribution.

Validation

• Focused child-device string-conversion tests: 2 passed
• Complete FoMaC C++ test binary: 171 passed
• uvx nox -s lint
• git diff --check

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the changed functionality.
• Documentation and migration guidance are not needed for this diagnostic-string completion.
• The existing changelog entry includes this PR and all human authors.
• The change follows the project's style guidelines and introduces no new warnings.
• The changes are fully tested and pass the relevant local checks.
• I have reviewed my own code changes.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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