Fix double-counted hyperspherical moment Jacobian

[FlorianPfaff]

Bug

AbstractHypersphereSubsetDistribution.moment_numerical() explicitly multiplied its hyperspherical-coordinate integrand by the surface-element factors (for example sin(phi2) on S2), then passed that already-weighted function to integrate_fun_over_domain(). The latter delegates to integrate_fun_over_domain_part(), which applies the hyperspherical surface-element Jacobian again.

This computes second moments against the wrong measure for dimensions >= 2. In particular, for a uniform distribution on S2, trace(E[x x^T]) should be 1 because ||x|| = 1, but the double Jacobian yields pi/4 instead.

Fix

Pass the Cartesian moment integrand in hyperspherical coordinates directly to integrate_fun_over_domain() and let the integration helper apply the surface Jacobian exactly once.

Regression test

Add a uniform-S2 identity check:

• E[x x^T] = I / 3
• trace(E[x x^T]) = 1

The test is skipped on JAX, consistent with the existing numerical hyperspherical integration limitation.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Max errors	Warnings	Elapsed time
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✅ PYTHON	black	2034	1	0		0	65.68s
✅ PYTHON	isort	2034	4	0		0	2.96s
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✅ REPOSITORY	checkov	yes		no		no	33.79s
✅ REPOSITORY	git_diff	yes		no		no	0.19s
✅ REPOSITORY	secretlint	yes		no		no	105.49s
✅ REPOSITORY	syft	yes		no		no	2.73s
✅ REPOSITORY	trivy-sbom	yes		no		no	3.05s
✅ YAML	prettier	11	0	0		0	0.61s
✅ YAML	v8r	11		0		0	16.81s
✅ YAML	yamllint	11		0		0	0.92s

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