[mayuriphad]

Fixes #986

solve_discrete_riccati_system loops while error > tolerance. When the iterates overflow, error becomes nan, and nan > tolerance is False. The loop then exits as if it had converged and returns non-finite Ps. Callers such as LQMarkov.stationary_values fail later with SciPy's array must not contain infs or NaNs, and the solver's own failure message (with the beta >= 1 stabilizability hint) is never raised. The doubling solver solve_discrete_riccati, used by LQ and Kalman, exits the same way.

Both loops now raise the existing fail_msg as soon as the error is not finite. The check adds no arithmetic, so runs that converge are unchanged.

Tests:

• Added test_solve_discrete_riccati_system_overflow_raises and test_solve_discrete_riccati_overflow_raises, both using the unstabilizable system x' = 2x, B = 0 from the issue. Both fail on main (no ValueError) and pass with this change.
• test_matrix_eqn, test_lqcontrol, test_ricatti, test_kalman, test_robustlq and test_lqnash: 47 passed.

Note: #984 changes the update inside the system solver's loop but not its exit condition. The new check sits just after the error sum, so any conflict between the two PRs should be trivial.

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[github-actions]

Thanks for the pull request, @mayuriphad!

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