Implement JAX positive-definite matrix check

[FlorianPfaff]

Summary

• implement a JAX-native backend.linalg.is_single_matrix_pd instead of routing it to NotImplementedError
• remove is_single_matrix_pd from the JAX linalg unsupported-capability metadata
• add subprocess regression coverage for real, complex Hermitian, indefinite, non-square, and non-Hermitian inputs under the JAX backend

Validation

• connector compare shows the changes are limited to src/pyrecest/_backend/jax/linalg.py, src/pyrecest/_backend/capabilities.py, and tests/backend_support/test_jax_linalg_contract.py
• local checkout/test execution was not possible in this sandbox because DNS resolution for github.com fails; PR CI should run the full matrix

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	26.0s
✅ PYTHON	black	3	1	0	0	1.4s
✅ PYTHON	isort	3	2	0	0	0.34s
✅ REPOSITORY	checkov	yes		no	no	26.51s
✅ REPOSITORY	gitleaks	yes		no	no	13.26s
✅ REPOSITORY	git_diff	yes		no	no	0.04s
✅ REPOSITORY	secretlint	yes		no	no	11.25s
✅ REPOSITORY	syft	yes		no	no	1.61s
✅ REPOSITORY	trivy-sbom	yes		no	no	2.67s
✅ REPOSITORY	trufflehog	yes		no	no	17.12s

Notices

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