Expose autograd in explicit backend helper

[FlorianPfaff]

Summary

• add autograd to SUPPORTED_BACKENDS in pyrecest.backends
• add a regression test that verifies get_backend("autograd") is accepted and resolves to pyrecest._backend.autograd

Notes

The public backend importer already supports PYRECEST_BACKEND=autograd, and the repository contains an autograd backend smoke test. This patch aligns the explicit backend access helper with that existing backend support.

Validation

• Regression coverage added in tests/test_explicit_backends.py
• Full CI left to GitHub Actions because the execution container cannot clone github.com directly from this session

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	56.35s
✅ JSON	prettier	7	0	0	0	1.08s
✅ JSON	v8r	7		0	0	5.62s
✅ MARKDOWN	markdownlint	66	0	0	0	1.68s
✅ MARKDOWN	markdown-table-formatter	66	0	0	0	0.9s
✅ PYTHON	black	784	0	0	0	42.8s
✅ PYTHON	isort	784	0	0	0	7.21s
✅ REPOSITORY	checkov	yes		no	no	50.7s
✅ REPOSITORY	gitleaks	yes		no	no	13.88s
✅ REPOSITORY	git_diff	yes		no	no	0.06s
✅ REPOSITORY	secretlint	yes		no	no	24.3s
✅ REPOSITORY	syft	yes		no	no	4.66s
✅ REPOSITORY	trivy-sbom	yes		no	no	5.84s
✅ REPOSITORY	trufflehog	yes		no	no	24.63s
✅ YAML	prettier	11	0	0	0	0.74s
✅ YAML	v8r	11		0	0	10.71s
✅ YAML	yamllint	11		0	0	0.55s

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