Fix association model solve fallback

[FlorianPfaff]

Summary

• make LogisticPairwiseAssociationModel's Newton solve fallback backend-neutral by retrying with pinv for backend runtime/value errors as well as NumPy LinAlgError
• retry with pinv when a backend solve returns non-finite steps, and fail explicitly if the fallback is also non-finite
• add focused regressions for raised backend solve errors and non-finite solve results

Rationale

The association model fit loop uses the active backend's linalg.solve, but previously only caught numpy.linalg.LinAlgError. Non-NumPy backends may signal singular or unsupported linear solves with different exception types, and some backend solve implementations can return non-finite values instead of raising. In both cases, the existing pseudoinverse fallback was bypassed and bad Newton steps could propagate into fitted parameters.

Validation

• Syntax-compiled the modified source and test file in the sandbox.
• Reviewed the branch diff through the GitHub connector: 2 files changed (src/pyrecest/utils/association_models.py, tests/test_association_models.py).
• Direct repository checkout and pytest execution were not possible here because the sandbox could not resolve github.com; PR CI should run the focused and full test matrix.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	37.34s
✅ PYTHON	black	2	2	0	0	2.68s
✅ PYTHON	isort	2	2	0	0	0.44s
✅ REPOSITORY	checkov	yes		no	no	32.09s
✅ REPOSITORY	gitleaks	yes		no	no	17.57s
✅ REPOSITORY	git_diff	yes		no	no	0.06s
✅ REPOSITORY	secretlint	yes		no	no	13.94s
✅ REPOSITORY	syft	yes		no	no	2.33s
✅ REPOSITORY	trivy-sbom	yes		no	no	3.84s
✅ REPOSITORY	trufflehog	yes		no	no	23.99s

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