Fix backend contract behavior

[FlorianPfaff]

Summary:

• fix backend reduction, take, cross, scatter_add, and slicing contract behavior
• add shared backend regression tests for the corrected contract behavior

Checks:

• git diff --check
• conflict marker search
• tests not run per request

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	32.09s
✅ PYTHON	black	2	2	0	0	2.14s
✅ PYTHON	isort	2	2	0	0	0.5s
✅ REPOSITORY	checkov	yes		no	no	32.31s
✅ REPOSITORY	gitleaks	yes		no	no	16.69s
✅ REPOSITORY	git_diff	yes		no	no	0.05s
✅ REPOSITORY	secretlint	yes		no	no	13.74s
✅ REPOSITORY	syft	yes		no	no	2.17s
✅ REPOSITORY	trivy-sbom	yes		no	no	3.58s
✅ REPOSITORY	trufflehog	yes		no	no	23.8s

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[github-actions]

Test Results

    10 files  ±0      10 suites  ±0   2h 34m 29s ⏱️ + 1m 58s
 2 005 tests ±0   2 001 ✅ ±0      4 💤 ±0  0 ❌ ±0 
20 475 runs  ±0  16 885 ✅ ±0  3 590 💤 ±0  0 ❌ ±0 

Results for commit 0c0670c. ± Comparison against base commit 346c3cb.

♻️ This comment has been updated with latest results.