Fix NaN validation to reject NaN for Parameters while allowing for Variables (#127)

Parameters should raise ValueError when assigned NaN values, but Variables
need to allow NaN for NLP structural Jacobian/Hessian computation. The
previous code allowed NaN for all Leaf types, breaking upstream test
test_nan_in_parameter_raises.

The fix checks self.variables() to distinguish Variables (returns [self])
from Parameters (returns []).

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: Transurgeon

cvxpy/expressions/leaf.py

@@ -601,9 +601,14 @@ def _validate_value(self, val, sparse_path=False):
                     attr_str = 'in bounds'
                 else:
                     attr_str = ([k for (k, v) in self.attributes.items() if v] + ['real'])[0]
-                if np.isnan(val).any():
+                if np.isnan(val).any() and self.variables():
                     # necessary for NLP package extension and computing the structural jacobian
+                    # Only allow NaN for Variables, not Parameters
                     return val
+                elif np.isnan(val).any():
+                    raise ValueError(
+                        "%s value must be real." % self.__class__.__name__
+                    )
                 else:
                     raise ValueError(
                        "%s value must be %s." % (self.__class__.__name__, attr_str)