[processing] Don't try to load algorithms for disabled providers

Fixes #18488

src/core/processing/qgsprocessingprovider.cpp

@@ -60,8 +60,11 @@ void QgsProcessingProvider::refreshAlgorithms()
 {
   qDeleteAll( mAlgorithms );
   mAlgorithms.clear();
-  loadAlgorithms();
-  emit algorithmsLoaded();
+  if ( isActive() )
+  {
+    loadAlgorithms();
+    emit algorithmsLoaded();
+  }
 }
 
 QList<const QgsProcessingAlgorithm *> QgsProcessingProvider::algorithms() const

tests/src/analysis/testqgsprocessing.cpp

@@ -323,6 +323,12 @@ class DummyProvider : public QgsProcessingProvider
       return supportsNonFileOutputs;
     }
 
+    bool isActive() const override
+    {
+      return active;
+    }
+
+    bool active = true;
     bool *unloaded = nullptr;
     bool supportsNonFileOutputs = false;
 
@@ -1079,6 +1085,16 @@ void TestQgsProcessing::algorithm()
     QCOMPARE( providerRefreshed.count(), 2 + i );
   }
 
+  // inactive provider, should not load algorithms
+  p->active = false;
+  p->refreshAlgorithms();
+  QCOMPARE( providerRefreshed.count(), 3 );
+  QVERIFY( p->algorithms().empty() );
+  p->active = true;
+  p->refreshAlgorithms();
+  QCOMPARE( providerRefreshed.count(), 4 );
+  QVERIFY( !p->algorithms().empty() );
+
   QgsProcessingRegistry r;
   r.addProvider( p );
   QCOMPARE( r.algorithms().size(), 2 );