Fix PyTorch Sylvester semidefinite denominator

[FlorianPfaff]

Summary

• Fix the PyTorch solve_sylvester real symmetric semidefinite shortcut so it only masks genuinely zero denominators instead of adding an identity matrix to every diagonal denominator.
• Add a regression test showing that near-skew right-hand sides accepted by the shortcut still satisfy the Sylvester residual tightly in non-null eigenspaces.

Testing

• Not run locally: the execution container cannot resolve github.com, so I used the GitHub connector for repository writes.
• Added tests/backend_support/test_pytorch_sylvester_semidefinite_shortcut.py, which runs the regression under PYRECEST_BACKEND=pytorch via the existing backend runner.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
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✅ JSON	v8r	7		0	0	7.06s
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✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.59s
✅ PYTHON	black	870	0	0	0	41.37s
✅ PYTHON	isort	870	0	0	0	6.81s
✅ REPOSITORY	checkov	yes		no	no	44.48s
✅ REPOSITORY	gitleaks	yes		no	no	9.14s
✅ REPOSITORY	git_diff	yes		no	no	0.12s
✅ REPOSITORY	secretlint	yes		no	no	26.7s
✅ REPOSITORY	syft	yes		no	no	4.61s
✅ REPOSITORY	trivy-sbom	yes		no	no	8.31s
✅ REPOSITORY	trufflehog	yes		no	no	16.39s
✅ YAML	prettier	11	0	0	0	0.66s
✅ YAML	v8r	11		0	0	10.52s
✅ YAML	yamllint	11		0	0	0.46s

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