fix: sampleCHRR returns NaN when the equality system is square

[edkerk]

Problem

sampleCHRR fails on a model as small as the one shipped with the documentation
site (52 metabolites, 53 reactions):

sampleChebyshevCenter: LP infeasible - flux polytope has empty interior.

The polytope is not empty. The error comes from NaNs created much earlier.

sampleCHRR folds implicitly-fixed reactions into the equality system:

Aeq = [S; blk];          % blk: one row per reaction with ~zero FVA width
v0  = Aeq \ beq;         % particular solution

On this model exactly one reaction is fixed (ethIN, bounds [0 0]), so Aeq
is 53 x 53 — square — with rank 44. For a rank-deficient square system
MATLAB's backslash returns NaN, where for a rectangular one it would return a
basic solution. The NaNs flow into A_full/b_full, and the failure only
surfaces later, in sampleChebyshevCenter, as an infeasible LP.

The trigger is therefore nMets + nFixed == nRxns, which is not exotic — it is
one arithmetic coincidence away on any model.

Fix

v0 = lsqminnorm(Aeq, beq);

lsqminnorm returns the minimum-norm solution for a rank-deficient system of
any shape. As the surrounding comment already notes, v0 may be any particular
solution — the sampled distribution does not depend on the choice — so this
changes nothing except the degenerate case. It also matches the Python
implementation, which uses lstsq.

Verification

On the same model, before: the error above at every fixedWidthTol tried
(1e-7, 1e-6, 1e-4). After:

9 dimensions, 1 fixed, 53 x 200 samples, no NaNs
growth mean 0.0460, range 0.0097..0.0992

The Python implementation reports 9 dimensions and 1 fixed reaction for the
same model, so the two now agree on the geometry; the sampled growth
distribution matches too (0.0098..0.0911 there, a different RNG).

randomSampling's default achr path is untouched and still returns NaN-free
samples.

Found while writing the sampling page of the new user guide.

[github-actions]

Function test results

300 tests   274 ✅  1m 7s ⏱️
 25 suites   26 💤
  1 files      0 ❌

Results for commit ba36fc1.