Fix scalar non-assignment costs in Murty assignments

[FlorianPfaff]

Summary

• Broadcast scalar and zero-dimensional array non-assignment costs to the required row/column length in murty_k_best_assignments.
• Keep vector-valued non-assignment costs unchanged, with a clearer validation message.
• Add regression coverage for Python scalar row costs and 0-D NumPy array column costs.

Rationale

row_non_assignment_costs=5.0 or col_non_assignment_costs=np.array(6.0) should behave as scalar broadcast costs. The old code reshaped scalar-like inputs to a length-one vector and rejected them whenever the corresponding problem dimension was not one.

Testing

• Added tests/utils/test_assignment.py::MurtyAssignmentTest::test_scalar_non_assignment_costs_are_broadcast.
• Full local pytest was not run because the execution container cannot resolve github.com; repository CI should run the focused assignment tests and full matrix.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	40.49s
✅ JSON	prettier	7	0	0	0	0.83s
✅ JSON	v8r	7		0	0	6.3s
✅ MARKDOWN	markdownlint	66	0	0	0	1.72s
✅ MARKDOWN	markdown-table-formatter	66	0	0	0	0.81s
✅ PYTHON	black	761	0	0	0	42.19s
✅ PYTHON	isort	761	0	0	0	4.73s
✅ REPOSITORY	checkov	yes		no	no	48.1s
✅ REPOSITORY	gitleaks	yes		no	no	17.32s
✅ REPOSITORY	git_diff	yes		no	no	0.18s
✅ REPOSITORY	secretlint	yes		no	no	23.45s
✅ REPOSITORY	syft	yes		no	no	7.71s
✅ REPOSITORY	trivy-sbom	yes		no	no	6.98s
✅ REPOSITORY	trufflehog	yes		no	no	22.83s
✅ YAML	prettier	11	0	0	0	0.76s
✅ YAML	v8r	11		0	0	12.25s
✅ YAML	yamllint	11		0	0	0.61s

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