Guard undefined hyperspherical mean directions

[FlorianPfaff]

Summary

• raise ValueError instead of normalizing a near-zero first moment in hyperspherical mean-direction computations
• apply the same guard to the hyperhemispherical numerical override
• add a regression test for the uniform circle, whose full-sphere mean direction is undefined

Rationale

For distributions with a zero or near-zero first moment, such as the full-circle uniform distribution, the mean direction is mathematically undefined. Previously the implementation emitted a warning and then divided by the near-zero norm anyway, which could yield NaNs or meaningless directions.

Testing

• Not run locally in this environment; changes are covered by a targeted regression test added to tests/distributions/test_abstract_hyperspherical_distribution.py.

[github-actions]

Test Results

    12 files  ± 0      12 suites  ±0   2h 22m 59s ⏱️ + 10m 9s
 1 834 tests + 1   1 833 ✅ +1      1 💤 ±0  0 ❌ ±0 
22 520 runs  +12  17 952 ✅ +8  4 568 💤 +4  0 ❌ ±0 

Results for commit 3b5f398. ± Comparison against base commit 7f90866.

♻️ This comment has been updated with latest results.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	22.13s
✅ JSON	prettier	2	0	0	0	0.45s
✅ JSON	v8r	2		0	0	3.01s
✅ MARKDOWN	markdownlint	28	0	0	0	1.45s
✅ MARKDOWN	markdown-table-formatter	28	0	0	0	0.36s
✅ PYTHON	bandit	571		0	0	10.18s
✅ PYTHON	black	571	29	0	0	19.28s
✅ PYTHON	flake8	571		0	0	6.33s
✅ PYTHON	isort	571	30	0	0	1.1s
✅ PYTHON	mypy	571		0	0	7.81s
✅ PYTHON	pylint	571		0	0	106.98s
✅ PYTHON	ruff	571	30	0	0	0.13s
✅ REPOSITORY	checkov	yes		no	no	27.68s
✅ REPOSITORY	gitleaks	yes		no	no	15.83s
✅ REPOSITORY	git_diff	yes		no	no	0.09s
✅ REPOSITORY	osv-scanner	yes		no	no	1.25s
✅ REPOSITORY	secretlint	yes		no	no	10.26s
✅ REPOSITORY	syft	yes		no	no	2.75s
✅ REPOSITORY	trivy-sbom	yes		no	no	4.05s
✅ REPOSITORY	trufflehog	yes		no	no	22.92s
✅ YAML	prettier	5	0	0	0	0.54s
✅ YAML	v8r	5		0	0	5.41s
✅ YAML	yamllint	5		0	0	0.49s

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