Fix PyTorch backend contract edge cases

[FlorianPfaff]

Summary

• tighten PyTorch backend contract handling for backend array helpers
• add focused backend contract coverage, including matmul behavior

Validation

• python -m py_compile src\pyrecest\_backend\pytorch\__init__.py tests\test_backend_contracts.py tests\backend_support\test_pytorch_matmul_contract.py
• git diff --check

Did not run tests per request.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	60.85s
✅ JSON	prettier	7	0	0	0	1.06s
✅ JSON	v8r	7		0	0	4.17s
✅ MARKDOWN	markdownlint	68	0	0	0	1.9s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.72s
✅ PYTHON	black	821	0	0	0	46.31s
✅ PYTHON	isort	821	1	0	0	4.41s
✅ REPOSITORY	checkov	yes		no	no	49.72s
✅ REPOSITORY	gitleaks	yes		no	no	14.15s
✅ REPOSITORY	git_diff	yes		no	no	0.06s
✅ REPOSITORY	secretlint	yes		no	no	24.98s
✅ REPOSITORY	syft	yes		no	no	3.53s
✅ REPOSITORY	trivy-sbom	yes		no	no	8.09s
✅ REPOSITORY	trufflehog	yes		no	no	25.52s
✅ YAML	prettier	11	0	0	0	0.72s
✅ YAML	v8r	11		0	0	11.18s
✅ YAML	yamllint	11		0	0	0.58s

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