Offer Use default on a node field only where there is one

The editor a node opens for a single field offered Use default on every
optional field that was not a checkbox: a storage's connection, its
operation, its scope, and any other choice between alternatives, where
there is nothing to go back to. The settings dialog already followed the
schema's x-ui-defaults-when-empty; this editor now does too, so the
button is on the model's sampling knobs and nowhere else.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/app/shared/nodes/generic-node/generic-node.spec.ts

@@ -857,6 +857,35 @@ describe('GenericNodeComponent', () => {
       expect(component.localEditorGlobalName).toBe('');
     });
 
+    it('offers Use default only on a field whose schema says it falls back to one', async () => {
+      // Every optional field used to get it - a choice between a catalog storage and your own
+      // connection, an operation, a scope - where there is nothing to go back to.
+      const component = fixture.componentInstance as any;
+      const choice = {
+        ...bindableFieldDefinition(),
+        path: 'source',
+        label: 'Connection',
+        enumOptions: ['CATALOG', 'PERSONAL'],
+        ui: { visibleWhen: [], enabledWhen: [] }
+      };
+      const knob = {
+        ...bindableFieldDefinition(),
+        path: 'llmDescriptor.parameters.temperature',
+        label: 'Temperature',
+        type: 'number' as const,
+        ui: { visibleWhen: [], enabledWhen: [], defaultsWhenEmpty: true }
+      };
+      component.editableFieldDefinitions = [choice, knob];
+      component.ensureBlockConfiguration()['llmDescriptor'] = { provider: 'InternalOllama' };
+
+      await component.openParameterEditor('source');
+      expect(component.showLocalEditorDefault()).toBe(false);
+
+      component.closeLocalEditor?.();
+      await component.openParameterEditor('llmDescriptor.parameters.temperature');
+      expect(component.showLocalEditorDefault()).toBe(true);
+    });
+
     it('renders the field description, which the modal never used to show', async () => {
       // The other half of why none of this was discoverable: the modal held no tip at all, so a
       // @UiDescription explaining that the field takes a placeholder was invisible.

src/app/shared/nodes/generic-node/generic-node.ts

@@ -213,7 +213,7 @@ export class GenericNodeComponent implements OnDestroy {
   localEditorPattern: string | null = null;
   /** The field's own description. The modal never rendered one, so every tip was invisible. */
   localEditorTip: string | null = null;
-  /** Empty means unset on this field, and the modal says so. Every optional field, not just numbers. */
+  /** Empty hands the field to a default, and the modal says so - only where the schema says it does. */
   localEditorDefaultsWhenEmpty = false;
   localEditorDefaultValue: string | null = null;
   localEditorMin: number | null = null;
@@ -449,8 +449,12 @@ export class GenericNodeComponent implements OnDestroy {
     this.localEditorMinLength = definition.ui.minLength ?? null;
     this.localEditorPattern = definition.ui.pattern ?? null;
     this.localEditorTip = definition.ui.tip ?? null;
-    // A boolean has no empty state: false is a value, not an absence.
-    this.localEditorDefaultsWhenEmpty = definition.type !== 'boolean' && !this.isPathRequired(definition.path);
+    // Only where the server says emptying the field hands it to a default - the model's sampling
+    // knobs. An optional field is not one: most have no default to go back to, and a choice
+    // between alternatives (a source, a scope, an operation) is picked, not reset. A boolean has
+    // no empty state at all: false is a value, not an absence.
+    this.localEditorDefaultsWhenEmpty = definition.ui.defaultsWhenEmpty === true
+      && definition.type !== 'boolean' && !this.isPathRequired(definition.path);
     this.localEditorDefaultValue = this.schemaDeclaredDefault(definition.path);
     this.localEditorMin = definition.ui.minimum ?? null;
     this.localEditorMax = definition.ui.maximum ?? null;