Fix JAX matmul out handling

[FlorianPfaff]

Summary

• add a JAX backend facade patch for matmul(..., out=...)
• align the public backend entry point and the JAX backend module entry point
• add regression coverage for compatible and incompatible out shapes

Testing

• Added tests/backend_support/test_jax_matmul_out_contract.py::test_jax_matmul_honors_out_shape_contract
• Syntax-checked the changed source and test content before committing; CI should run the full test suite.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	74.18s
✅ JSON	prettier	7	0	0	0	1.07s
✅ JSON	v8r	7		0	0	3.91s
✅ MARKDOWN	markdownlint	68	0	0	0	1.84s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.82s
✅ PYTHON	black	1095	0	0	0	61.6s
✅ PYTHON	isort	1095	0	0	0	4.57s
✅ REPOSITORY	checkov	yes		no	no	47.29s
✅ REPOSITORY	gitleaks	yes		no	no	10.95s
✅ REPOSITORY	git_diff	yes		no	no	0.09s
✅ REPOSITORY	secretlint	yes		no	no	37.39s
✅ REPOSITORY	syft	yes		no	no	4.5s
✅ REPOSITORY	trivy-sbom	yes		no	no	12.51s
✅ REPOSITORY	trufflehog	yes		no	no	19.91s
✅ YAML	prettier	11	0	0	0	0.75s
✅ YAML	v8r	11		0	0	12.83s
✅ YAML	yamllint	11		0	0	0.72s

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