fix(chat): do not delete the answer when regenerating without a selected model

regenerateResponse removed the previous answer before checking for a model and then returned silently, leaving the conversation without a reply. It now checks first and explains that a model is required, like sendMessage. The duplicated streaming code in sendMessage and regenerateResponse is now a single streamAnswer helper.

__tests__/useLMStudio.test.ts

@@ -177,22 +177,34 @@ describe('regenerateResponse', () => {
         expect(state().isStreaming).toBe(true);
     });
 
-    it('does nothing without a user message, while streaming, or without a model', async () => {
+    it('keeps the current answer and asks for a model when none is selected', async () => {
+        state().addMessage({ role: 'user', content: 'q' });
+        state().addMessage({ role: 'assistant', content: 'a' });
+        useAppStore.setState({ activeModelId: '' });
+        const { result } = renderHook(() => useLMStudio());
+
+        await act(async () => {
+            await result.current.regenerateResponse();
+        });
+        expect(mockApi.chatCompletionStream).not.toHaveBeenCalled();
+        expect(state().messages.map((m) => m.content)).toEqual([
+            'q',
+            'a',
+            expect.stringContaining('No model selected'),
+        ]);
+    });
+
+    it('does nothing without a user message or while streaming', async () => {
         const { result, rerender } = renderHook(() => useLMStudio());
         await act(async () => {
             await result.current.regenerateResponse();
         });
         expect(mockApi.chatCompletionStream).not.toHaveBeenCalled();
+        expect(state().messages).toEqual([]);
 
         act(() => {
             state().addMessage({ role: 'user', content: 'q' });
-            useAppStore.setState({ activeModelId: '' });
         });
-        await act(async () => {
-            await result.current.regenerateResponse();
-        });
-        expect(mockApi.chatCompletionStream).not.toHaveBeenCalled();
-
         act(() => {
             useAppStore.setState({ activeModelId: 'model-a', isStreaming: true });
         });