fix: set PV_MUMC_biexp fallback params to 0,0,0 on optimizer failure

Author: Devguru-codes

src/standardized/PV_MUMC_biexp.py

@@ -81,7 +81,7 @@ def ivim_fit(self, signals, bvalues=None):
             self.thresholds = 200
 
         # Default fallback parameters (D, f, Dp) used if the optimizer fails
-        DEFAULT_PARAMS = [0.003, 0.1, 0.05]
+        DEFAULT_PARAMS = [0, 0, 0]
 
         try:
             fit_results = self.PV_algorithm(bvalues, signals, bounds=bounds, cutoff=self.thresholds)