[processing] Fix "Centroids" alg with multipart geometries

when the "Create centroid for each part" option is enabled discard incoming fid field and regenerate it

See 1b2ca79 #42363

src/analysis/processing/qgsalgorithmcentroid.cpp

@@ -50,6 +50,14 @@ QString QgsCentroidAlgorithm::outputName() const
   return QObject::tr( "Centroids" );
 }
 
+QgsFeatureSink::SinkFlags QgsCentroidAlgorithm::sinkFlags() const
+{
+  if ( mAllParts )
+    return QgsProcessingFeatureBasedAlgorithm::sinkFlags() | QgsFeatureSink::RegeneratePrimaryKey;
+  else
+    return QgsProcessingFeatureBasedAlgorithm::sinkFlags();
+}
+
 QString QgsCentroidAlgorithm::shortHelpString() const
 {
   return QObject::tr( "This algorithm creates a new point layer, with points representing the centroid of the geometries in an input layer.\n\n"

src/analysis/processing/qgsalgorithmcentroid.h

@@ -51,6 +51,7 @@ class QgsCentroidAlgorithm : public QgsProcessingFeatureBasedAlgorithm
     QString outputName() const override;
     QgsProcessing::SourceType outputLayerType() const override { return QgsProcessing::TypeVectorPoint; }
     QgsWkbTypes::Type outputWkbType( QgsWkbTypes::Type inputWkbType ) const override { Q_UNUSED( inputWkbType ) return QgsWkbTypes::Point; }
+    QgsFeatureSink::SinkFlags sinkFlags() const override;
 
     bool prepareAlgorithm( const QVariantMap &parameters, QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
     QgsFeatureList processFeature( const QgsFeature &feature,  QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;