Implement JAX custom gradient support

[FlorianPfaff]

Summary

• implement JAX backend custom_gradient through jax.custom_vjp
• preserve keyword/default argument binding for custom gradients
• add JAX autodiff regression coverage

Checks

• Not run per request.
• python -m py_compile src\pyrecest_backend\jax\autodiff.py tests\test_jax_autodiff_backend.py
• git diff --check

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	52.61s
✅ JSON	prettier	7	0	0	0	0.86s
✅ JSON	v8r	7		0	0	4.97s
✅ MARKDOWN	markdownlint	68	0	0	0	1.67s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.67s
✅ PYTHON	black	827	5	0	0	46.88s
✅ PYTHON	isort	827	7	0	0	2.73s
✅ REPOSITORY	checkov	yes		no	no	43.31s
✅ REPOSITORY	gitleaks	yes		no	no	13.51s
✅ REPOSITORY	git_diff	yes		no	no	0.17s
✅ REPOSITORY	secretlint	yes		no	no	24.28s
✅ REPOSITORY	syft	yes		no	no	4.73s
✅ REPOSITORY	trivy-sbom	yes		no	no	5.68s
✅ REPOSITORY	trufflehog	yes		no	no	24.6s
✅ YAML	prettier	11	0	0	0	0.72s
✅ YAML	v8r	11		0	0	11.17s
✅ YAML	yamllint	11		0	0	0.51s

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