Validate candidate pruning scalar controls

[FlorianPfaff]

Summary

• Validate candidate-pruning scalar controls through finite-scalar helpers instead of raw float(...) coercion.
• Reject booleans and non-scalar arrays for probability_threshold, max_cost, max_cost_percentile, large_cost, and the large_cost override.
• Preserve existing probability-matrix semantics; this PR only fixes scalar control validation.

Testing

• Added focused regression coverage in tests/test_candidate_pruning.py.

Note: I did not run the tests locally because repository access here is through the GitHub connector.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	46.29s
✅ JSON	prettier	7	0	0	0	0.87s
✅ JSON	v8r	7		0	0	3.53s
✅ MARKDOWN	markdownlint	68	0	0	0	1.35s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.62s
✅ PYTHON	black	890	10	0	0	38.14s
✅ PYTHON	isort	890	11	0	0	2.31s
✅ REPOSITORY	checkov	yes		no	no	33.86s
✅ REPOSITORY	gitleaks	yes		no	no	7.21s
✅ REPOSITORY	git_diff	yes		no	no	0.09s
✅ REPOSITORY	secretlint	yes		no	no	20.74s
✅ REPOSITORY	syft	yes		no	no	2.34s
✅ REPOSITORY	trivy-sbom	yes		no	no	8.5s
✅ REPOSITORY	trufflehog	yes		no	no	14.14s
✅ YAML	prettier	11	0	0	0	0.56s
✅ YAML	v8r	11		0	0	8.48s
✅ YAML	yamllint	11		0	0	0.38s

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