fix(coding): preserve runtime model and failure contracts

tests/unit/coding-core-routes.test.ts

@@ -187,6 +187,57 @@ describe('PI-100 coding core Host contract', () => {
     });
   });
 
+  it('switches a resolved active Conversation model through the target runtime without disposing it', async () => {
+    const result = await setup();
+    const conversation = await createConversation(result.conversations);
+    await result.conversations.acceptPrompt({
+      conversationId: conversation.id,
+      clientRequestId: 'request-active-model-switch',
+      mode: 'prompt',
+      text: 'Keep this run active',
+      attachments: [],
+    });
+    await expect(result.runtime.getSnapshot(conversation.id)).resolves.toMatchObject({
+      run: { status: 'running' },
+    });
+    const setModel = vi.spyOn(result.runtime, 'setModel');
+    const dispose = vi.spyOn(result.runtime, 'dispose');
+    const nextModel = { ...MODEL, modelId: 'model-next' };
+
+    await expect(result.conversations.setModel(conversation.id, nextModel)).resolves.toEqual({
+      model: nextModel,
+      modelResolution: 'resolved',
+    });
+    expect(setModel).toHaveBeenCalledWith({
+      conversationId: conversation.id,
+      accountId: nextModel.accountId,
+      modelId: nextModel.modelId,
+    });
+    expect(dispose).not.toHaveBeenCalled();
+    await expect(result.runtime.getSnapshot(conversation.id)).resolves.toMatchObject({
+      run: { status: 'running' },
+    });
+    await expect(result.projects.conversationStore(result.root).get(conversation.id)).resolves.toMatchObject({
+      model: nextModel,
+      modelResolution: 'resolved',
+    });
+  });
+
+  it('maps Conversation thinking metadata write failures to the stable storage error', async () => {
+    const result = await setup();
+    const conversation = await createConversation(result.conversations);
+    await result.conversations.getSnapshot(conversation.id);
+    const store = result.projects.conversationStore(result.root);
+    vi.spyOn(result.projects, 'conversationStore').mockReturnValue(store);
+    vi.spyOn(store, 'setModelState')
+      .mockRejectedValueOnce(new Error('disk full'));
+
+    await expect(result.conversations.setThinking(conversation.id, 'high')).rejects.toMatchObject({
+      status: 500,
+      code: 'CODING_STORAGE_WRITE_FAILED',
+    });
+  });
+
   it('disposes and moves a bound session to Main-owned trash before deleting metadata', async () => {
     const result = await setup();
     const conversation = await createConversation(result.conversations);