Fix Pareto front eligibility for missing objectives

[FlorianPfaff]

Summary

• exclude candidates with no comparable objective values from Pareto-front membership when missing values are allowed
• require complete objective values for front eligibility when allow_missing=False
• add regression coverage for both pareto_front_indices and is_pareto_front

Testing

• Validated the changed Pareto helper logic locally with a minimal pandas/NumPy reproduction
• Full repository test suite not run in this environment

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
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✅ JSON	prettier	7	0	0	0	1.15s
✅ JSON	v8r	7		0	0	5.45s
✅ MARKDOWN	markdownlint	68	0	0	0	2.7s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.82s
✅ PYTHON	black	1132	5	0	0	63.43s
✅ PYTHON	isort	1132	5	0	0	3.18s
✅ REPOSITORY	checkov	yes		no	no	49.98s
✅ REPOSITORY	gitleaks	yes		no	no	12.34s
✅ REPOSITORY	git_diff	yes		no	no	0.23s
✅ REPOSITORY	secretlint	yes		no	no	45.41s
✅ REPOSITORY	syft	yes		no	no	4.83s
✅ REPOSITORY	trivy-sbom	yes		no	no	7.18s
✅ REPOSITORY	trufflehog	yes		no	no	17.87s
✅ YAML	prettier	11	0	0	0	0.78s
✅ YAML	v8r	11		0	0	11.79s
✅ YAML	yamllint	11		0	0	0.57s

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