Fix PyTorch stack helpers with NumPy backend

[FlorianPfaff]

Summary

• Patch the low-level PyTorch stack helper entry points from the backend support compatibility hook.
• Preserve NumPy-style array-like sequence support even when the selected public backend is NumPy.
• Add a subprocess regression for hstack, vstack, column_stack, and dstack with PYRECEST_BACKEND=numpy.

Bug fixed

When the process selected the NumPy public backend, the PyTorch-specific stack-helper normalization path did not run. Direct PyTorch backend helper calls could therefore still reach Torch-native stack functions, which reject Python list elements such as ([1, 2], [3, 4]).

Testing

• Added tests/backend_support/test_pytorch_stack_helpers_raw_backend_contract.py.
• Not run locally in this connector-only session; CI should execute the focused backend-portable regression.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	74.59s
✅ JSON	prettier	7	0	0	0	0.75s
✅ JSON	v8r	7		0	0	4.04s
✅ MARKDOWN	markdownlint	68	0	0	0	1.93s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.8s
✅ PYTHON	black	1132	4	0	0	63.88s
✅ PYTHON	isort	1132	4	0	0	4.48s
✅ REPOSITORY	checkov	yes		no	no	49.64s
✅ REPOSITORY	gitleaks	yes		no	no	8.93s
✅ REPOSITORY	git_diff	yes		no	no	0.1s
✅ REPOSITORY	secretlint	yes		no	no	32.83s
✅ REPOSITORY	syft	yes		no	no	4.0s
✅ REPOSITORY	trivy-sbom	yes		no	no	11.65s
✅ REPOSITORY	trufflehog	yes		no	no	21.72s
✅ YAML	prettier	11	0	0	0	0.75s
✅ YAML	v8r	11		0	0	12.45s
✅ YAML	yamllint	11		0	0	0.64s

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