[codex] Preserve typed artifact prediction labels

[FlorianPfaff]

Summary

• Keep semantic fields such as true_label in the typed prediction-row form created by _prediction_row instead of overwriting them with key-column CSV strings.
• This addresses the remote unit-test failure where balanced-assignment prediction rows returned '1' instead of 1 for true_label.

Validation

• Not run locally per request; waiting for GitHub checks.

[github-actions]

✅⚠️MegaLinter analysis: Success with warnings

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	3.41s
⚠️ MARKDOWN	markdownlint	17	0	1	0	0.75s
✅ PYTHON	ruff	149	0	0	0	0.39s
✅ REPOSITORY	git_diff	yes		no	no	0.03s
✅ YAML	prettier	7	0	0	0	0.48s
✅ YAML	v8r	7		0	0	3.51s
✅ YAML	yamllint	7		0	0	0.37s

Detailed Issues

⚠️ MARKDOWN / markdownlint - 1 error

.github/PULL_REQUEST_TEMPLATE.md:1 error MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading [Context: "## Archive-scope checklist"]

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