Validate piecewise numerical PDF outputs

[FlorianPfaff]

Summary

• Validate each pdf_func value returned to PiecewiseConstantDistribution.calculate_parameters_numerically before passing it to SciPy quadrature.
• Accept scalar and single-element outputs, but reject empty or multi-element outputs instead of silently using the first flattened value.
• Reject non-finite numerical PDF outputs and add regression coverage for scalar, malformed, and non-finite returns.

Bug fixed

calculate_parameters_numerically() previously evaluated float(array(pdf_func(array([x]))).reshape(-1)[0]). If a caller supplied a malformed pdf_func that returned an empty vector, the error was obscure; if it returned multiple values for one integration point, the routine silently integrated only the first value and ignored the rest. That can produce misleading discretized weights from an invalid PDF contract.

Validation

• Syntax-checked the modified source and test module with ast.parse locally.
• Ran an isolated local harness for the new scalar-output validation behavior: scalar output integrates to uniform weights, empty/vector outputs raise ValueError, and NaN output raises ValueError.
• Full repository pytest was not run locally because the sandbox cannot resolve github.com to clone/install the full repository dependency set.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	72.78s
✅ JSON	prettier	7	0	0	0	1.05s
✅ JSON	v8r	7		0	0	5.53s
✅ MARKDOWN	markdownlint	68	0	0	0	1.74s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.91s
✅ PYTHON	black	1036	38	0	0	65.8s
✅ PYTHON	isort	1036	56	0	0	4.24s
✅ REPOSITORY	checkov	yes		no	no	51.02s
✅ REPOSITORY	gitleaks	yes		no	no	7.56s
✅ REPOSITORY	git_diff	yes		no	no	0.1s
✅ REPOSITORY	secretlint	yes		no	no	31.28s
✅ REPOSITORY	syft	yes		no	no	3.77s
✅ REPOSITORY	trivy-sbom	yes		no	no	11.55s
✅ REPOSITORY	trufflehog	yes		no	no	21.47s
✅ YAML	prettier	11	0	0	0	0.92s
✅ YAML	v8r	11		0	0	10.36s
✅ YAML	yamllint	11		0	0	0.54s

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