Fix JAX assignment indexing

[FlorianPfaff]

Summary

• normalize JAX backend assignment indices before applying .at[...], so list indices, boolean masks, and list-of-coordinate advanced indices work consistently with the intended NumPy-style backend contract
• preserve vectorized shorter-index assignment semantics and value-count validation
• add JAX subprocess regression coverage for assignment and assignment_by_sum

Validation

• reproduced the raw JAX failure locally for list/list-of-tuple indices via jax.numpy (TypeError: Using a non-tuple sequence for multidimensional indexing is not allowed)
• locally exercised the normalization behavior for list scalar indices, coordinate tuples, boolean masks, and assignment_by_sum
• connector diff check: changes are limited to src/pyrecest/_backend/jax/__init__.py and tests/backend_support/test_jax_assignment_contract.py

Note: I could not run the repository pytest suite locally because this sandbox cannot resolve github.com for checkout; PR CI should run the suite.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	36.78s
✅ PYTHON	black	2	1	0	0	2.58s
✅ PYTHON	isort	2	2	0	0	0.6s
✅ REPOSITORY	checkov	yes		no	no	32.44s
✅ REPOSITORY	gitleaks	yes		no	no	17.33s
✅ REPOSITORY	git_diff	yes		no	no	0.07s
✅ REPOSITORY	secretlint	yes		no	no	13.24s
✅ REPOSITORY	syft	yes		no	no	2.73s
✅ REPOSITORY	trivy-sbom	yes		no	no	4.31s
✅ REPOSITORY	trufflehog	yes		no	no	23.47s

Notices

📣 MegaLinter 9.5.0 is out! Discover the new features and security recommendations in the release announcement. (Skip this info by defining SECURITY_SUGGESTIONS: false)

See detailed reports in MegaLinter artifacts
Set VALIDATE_ALL_CODEBASE: true in mega-linter.yml to validate all sources, not only the diff

Show us your support by starring ⭐ the repository