fix: raise ValueError when run_simplex exhausts maxiter

[SAY-5]

Describe your change:

Tableau.run_simplex was silently returning an empty dict {} after exhausting Tableau.maxiter iterations, making it impossible for callers to tell convergence failure apart from a problem whose optimum is genuinely all-zero. The empty-dict path also masked cycling/unboundedness.

Now it raises ValueError with the iteration limit so the caller gets an actionable signal. All existing doctests still pass.

Fixes #14584

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