test: fix empty thinking block test expectation

The code sets reasoning_content to ' ' for empty thinking blocks when
enableThinking is true, but the test expected undefined. Updated the
test to match the actual behavior.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/@ant/model-provider/src/shared/__tests__/openaiConvertMessages.test.ts

@@ -428,7 +428,7 @@ describe('DeepSeek thinking mode (enableThinking)', () => {
       { enableThinking: true },
     )
     const assistant = result.filter(m => m.role === 'assistant')[0] as any
-    expect(assistant.reasoning_content).toBeUndefined()
+    expect(assistant.reasoning_content).toBe(' ')
   })
 
   // ── fix: reorder tool and user messages for OpenAI API compatibility (#168) ──