Fix backend and scalar parameter handling

[FlorianPfaff]

Summary

• harden JAX and PyTorch backend shape, assignment, logical, and random uniform behavior
• accept scalar parameters in wrapped and toroidal distributions
• add backend contract and scalar-parameter regression coverage

Validation

• Ran git diff --check only.
• Tests not run per request.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	35.95s
✅ PYTHON	black	8	1	0	0	3.63s
✅ PYTHON	isort	8	1	0	0	0.54s
✅ REPOSITORY	checkov	yes		no	no	33.27s
✅ REPOSITORY	gitleaks	yes		no	no	18.11s
✅ REPOSITORY	git_diff	yes		no	no	0.06s
✅ REPOSITORY	secretlint	yes		no	no	14.22s
✅ REPOSITORY	syft	yes		no	no	2.89s
✅ REPOSITORY	trivy-sbom	yes		no	no	4.6s
✅ REPOSITORY	trufflehog	yes		no	no	24.26s

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