Fix JAX sparse array flat indexing

[FlorianPfaff]

Summary

• Fix array_from_sparse in the JAX backend by applying raveled indices to a flattened dense array before reshaping back to the requested target shape.
• Add regression tests for 2-D and 3-D multidimensional sparse-to-dense conversion.

Why

The previous implementation computed flat/raveled indices with jnp.ravel_multi_index, but then used those positions to index the original n-D output array. For multidimensional target shapes this indexes the first axis rather than the flattened storage order, which can write values to the wrong positions or raise out-of-bounds errors.

Tests

• Not run locally; change was made through the GitHub connector.
• Added tests/backend/test_jax_array_from_sparse.py for CI coverage.

[github-actions]

❌MegaLinter analysis: Error

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	16.97s
✅ JSON	prettier	2	0	0	0	0.43s
✅ JSON	v8r	2		0	0	2.77s
✅ MARKDOWN	markdownlint	28	0	0	0	1.31s
✅ MARKDOWN	markdown-table-formatter	28	0	0	0	0.33s
✅ PYTHON	bandit	575		0	0	9.15s
✅ PYTHON	black	575	1	0	0	14.27s
✅ PYTHON	flake8	575		0	0	5.33s
✅ PYTHON	isort	575	2	0	0	1.02s
✅ PYTHON	mypy	575		0	0	6.95s
✅ PYTHON	pylint	575		0	0	97.57s
✅ PYTHON	ruff	575	2	0	0	0.11s
✅ REPOSITORY	checkov	yes		no	no	25.33s
✅ REPOSITORY	gitleaks	yes		no	no	14.16s
✅ REPOSITORY	git_diff	yes		no	no	0.09s
❌ REPOSITORY	osv-scanner	yes		3	no	1.05s
✅ REPOSITORY	secretlint	yes		no	no	9.55s
✅ REPOSITORY	syft	yes		no	no	3.17s
✅ REPOSITORY	trivy-sbom	yes		no	no	3.57s
✅ REPOSITORY	trufflehog	yes		no	no	20.8s
✅ YAML	prettier	5	0	0	0	0.5s
✅ YAML	v8r	5		0	0	4.9s
✅ YAML	yamllint	5		0	0	0.44s

Detailed Issues

❌ REPOSITORY / osv-scanner - 3 errors

Scanning dir .
Starting filesystem walk for root: /
Scanned requirements.txt file and found 63 packages
Scanned requirements-dev.txt file and found 75 packages
Scanned poetry.lock file and found 89 packages
End status: 45 dirs visited, 719 inodes visited, 3 Extract calls, 75.627155ms elapsed, 75.627315ms wall time

Total 3 packages affected by 3 known vulnerabilities (0 Critical, 0 High, 3 Medium, 0 Low, 0 Unknown) from 1 ecosystem.
3 vulnerabilities can be fixed.

+-------------------------------------+------+-----------+---------+---------+---------------+----------------------+
| OSV URL                             | CVSS | ECOSYSTEM | PACKAGE | VERSION | FIXED VERSION | SOURCE               |
+-------------------------------------+------+-----------+---------+---------+---------------+----------------------+
| https://osv.dev/GHSA-65pc-fj4g-8rjx | 6.9  | PyPI      | idna    | 3.13    | 3.15          | poetry.lock          |
| https://osv.dev/GHSA-65pc-fj4g-8rjx | 6.9  | PyPI      | idna    | 3.13    | 3.15          | requirements-dev.txt |
| https://osv.dev/GHSA-65pc-fj4g-8rjx | 6.9  | PyPI      | idna    | 3.13    | 3.15          | requirements.txt     |
+-------------------------------------+------+-----------+---------+---------+---------------+----------------------+

Notices

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[github-actions]

Test Results

0 tests  ±0   0 ✅ ±0   0s ⏱️ ±0s
0 suites ±0   0 💤 ±0 
0 files   ±0   0 ❌ ±0 

Results for commit ea7dc2b. ± Comparison against base commit a91f29c.