Fix PyTorch multivariate_normal array-like inputs

[FlorianPfaff]

Summary

• coerce PyTorch random.multivariate_normal mean/covariance arguments to tensors before constructing torch.distributions.MultivariateNormal
• preserve tensor device placement and floating precision when either argument is already a tensor
• add backend-random regression coverage for NumPy-style Python sequence inputs

Rationale

The NumPy and JAX random backends accept ordinary array-like inputs for random.multivariate_normal, e.g. random.multivariate_normal([0.0, 0.0], [[1.0, 0.0], [0.0, 1.0]], size=(6,)). The PyTorch backend passed these lists directly into torch.distributions.MultivariateNormal, which fails because PyTorch expects tensor arguments.

Validation

• Connector compare confirms this branch is two commits ahead of current main and behind by zero.
• Changes are limited to src/pyrecest/_backend/pytorch/random.py and tests/test_backend_random.py.
• Local smoke test of the new PyTorch conversion helper with list inputs and tensor dtype preservation passed in the sandbox's installed PyTorch.
• Full repository tests were not run because the sandbox cannot clone/install the repository from GitHub; PR CI should run the backend matrix.

[github-actions]

✅⚠️MegaLinter analysis: Success with warnings

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	35.15s
✅ JSON	prettier	7	0	0	0	0.92s
✅ JSON	v8r	7		0	0	4.98s
⚠️ MARKDOWN	markdownlint	66	0	1	0	1.8s
✅ MARKDOWN	markdown-table-formatter	66	0	0	0	0.79s
✅ PYTHON	black	758	3	0	0	36.94s
✅ PYTHON	isort	758	3	0	0	2.17s
✅ REPOSITORY	checkov	yes		no	no	34.76s
✅ REPOSITORY	gitleaks	yes		no	no	14.52s
✅ REPOSITORY	git_diff	yes		no	no	0.09s
✅ REPOSITORY	secretlint	yes		no	no	24.09s
✅ REPOSITORY	syft	yes		no	no	6.62s
✅ REPOSITORY	trivy-sbom	yes		no	no	5.47s
✅ REPOSITORY	trufflehog	yes		no	no	19.89s
✅ YAML	prettier	10	0	0	0	0.65s
✅ YAML	v8r	10		0	0	10.43s
✅ YAML	yamllint	10		0	0	0.44s

Detailed Issues

⚠️ MARKDOWN / markdownlint - 1 error

.github/pull_request_template.md:1 error MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading [Context: "## Summary"]

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