Fix PyTorch random.multinomial sequence inputs

[FlorianPfaff]

Summary

• coerce PyTorch backend random.multinomial probability inputs to tensors before normalization
• preserve tensor device placement when pvals is already a tensor
• add backend-random regression coverage for Python probability sequences

Rationale

The NumPy and JAX random backends accept ordinary Python probability sequences for random.multinomial. The PyTorch backend assumed pvals was already a tensor and called .sum() directly, so portable code such as random.multinomial(12, [0.25, 0.75]) failed only under PYRECEST_BACKEND=pytorch.

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
• direct local pytest execution was not available through the GitHub connector; 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	42.28s
✅ JSON	prettier	7	0	0	0	0.96s
✅ JSON	v8r	7		0	0	5.03s
⚠️ MARKDOWN	markdownlint	66	0	2	0	1.81s
✅ MARKDOWN	markdown-table-formatter	66	1	0	0	0.59s
✅ PYTHON	black	758	2	0	0	34.55s
✅ PYTHON	isort	758	2	0	0	2.14s
✅ REPOSITORY	checkov	yes		no	no	41.21s
✅ REPOSITORY	gitleaks	yes		no	no	15.89s
✅ REPOSITORY	git_diff	yes		no	no	0.1s
✅ REPOSITORY	secretlint	yes		no	no	20.94s
✅ REPOSITORY	syft	yes		no	no	4.63s
✅ REPOSITORY	trivy-sbom	yes		no	no	5.61s
✅ REPOSITORY	trufflehog	yes		no	no	21.45s
✅ YAML	prettier	10	0	0	0	0.61s
✅ YAML	v8r	10		0	0	10.37s
✅ YAML	yamllint	10		0	0	0.4s

Detailed Issues

⚠️ MARKDOWN / markdownlint - 2 errors

.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"]
docs/backend-support.md:11:174 error MD060/table-column-style Table column style [Table pipe does not align with header for style "aligned"]

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