Fix PyTorch concatenate axis None contract

[FlorianPfaff]

Summary

• Patch the PyTorch backend compatibility layer so concatenate(..., axis=None) flattens each input and concatenates along axis 0, matching NumPy semantics instead of forwarding None to torch.cat.
• Apply the patch to both the raw PyTorch backend and the public backend when PYRECEST_BACKEND=pytorch.
• Add regression coverage for the raw PyTorch backend after pyrecest import and the public PyTorch backend.

Fixes #3677.

Tests

• CI passed for the PR head commit, including the main test workflow, security checks, dependency review, documentation examples, CodeQL, release notes preview, and MegaLinter.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	72.29s
✅ JSON	prettier	7	0	0	0	1.34s
✅ JSON	v8r	7		0	0	4.25s
✅ MARKDOWN	markdownlint	68	0	0	0	1.8s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.88s
✅ PYTHON	black	1207	37	0	0	61.09s
✅ PYTHON	isort	1207	72	0	0	3.37s
✅ REPOSITORY	checkov	yes		no	no	46.17s
✅ REPOSITORY	gitleaks	yes		no	no	11.48s
✅ REPOSITORY	git_diff	yes		no	no	0.31s
✅ REPOSITORY	secretlint	yes		no	no	48.77s
✅ REPOSITORY	syft	yes		no	no	4.99s
✅ REPOSITORY	trivy-sbom	yes		no	no	5.37s
✅ REPOSITORY	trufflehog	yes		no	no	15.7s
✅ YAML	prettier	11	0	0	0	0.68s
✅ YAML	v8r	11		0	0	10.77s
✅ YAML	yamllint	11		0	0	0.51s

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