fix(core): handle missing delta in OpenAI stream chunks

Some OpenAI-compatible providers occasionally emit chat.completion.chunk choices
without a delta object. Guard optional reasoning_content access and add a
regression test to ensure chunk conversion does not throw.

packages/core/src/core/openaiContentGenerator/converter.test.ts

@@ -207,6 +207,27 @@ describe('OpenAIContentConverter', () => {
         expect.objectContaining({ text: 'visible text' }),
       );
     });
+
+    it('should not throw when streaming chunk has no delta', () => {
+      const chunk = converter.convertOpenAIChunkToGemini({
+        object: 'chat.completion.chunk',
+        id: 'chunk-2',
+        created: 456,
+        choices: [
+          {
+            index: 0,
+            // Some OpenAI-compatible providers may omit delta entirely.
+            delta: undefined,
+            finish_reason: null,
+            logprobs: null,
+          },
+        ],
+        model: 'gpt-test',
+      } as unknown as OpenAI.Chat.ChatCompletionChunk);
+
+      const parts = chunk.candidates?.[0]?.content?.parts;
+      expect(parts).toEqual([]);
+    });
   });
 
   describe('convertGeminiToolsToOpenAI', () => {

packages/core/src/core/openaiContentGenerator/converter.ts

@@ -791,7 +791,7 @@ export class OpenAIContentConverter {
       const parts: Part[] = [];
 
       const reasoningText = (choice.delta as ExtendedCompletionChunkDelta)
-        .reasoning_content;
+        ?.reasoning_content;
       if (reasoningText) {
         parts.push({ text: reasoningText, thought: true });
       }

packages/core/src/services/shellExecutionService.test.ts

@@ -818,7 +818,7 @@ describe('ShellExecutionService child_process fallback', () => {
   });
 
   describe('Platform-Specific Behavior', () => {
-    it('should use cmd.exe on Windows', async () => {
+    it('should use cmd.exe and hide window on Windows', async () => {
       mockPlatform.mockReturnValue('win32');
       await simulateExecution('dir "foo bar"', (cp) =>
         cp.emit('exit', 0, null),
@@ -829,7 +829,8 @@ describe('ShellExecutionService child_process fallback', () => {
         [],
         expect.objectContaining({
           shell: true,
-          detached: true,
+          detached: false,
+          windowsHide: true,
         }),
       );
     });

packages/core/src/services/shellExecutionService.ts

@@ -229,7 +229,8 @@ export class ShellExecutionService {
         stdio: ['ignore', 'pipe', 'pipe'],
         windowsVerbatimArguments: true,
         shell: isWindows ? true : 'bash',
-        detached: true,
+        detached: !isWindows,
+        windowsHide: isWindows,
         env: {
           ...process.env,
           QWEN_CODE: '1',