fix(ai): ensure warnings promise in streamObject is resolved with correct warnings (#8133)

## Background
The `warnings` promise on `streamObject()` never resolves. Awaiting on
it results in the program hanging.

## Summary
- Added two unit tests for no warnings and some warnings. The unit tests
both cause the unit tests to timeout (current broken behavior)
- Make sure to resolve the warnings promise
- Enqueue chunks for `stream-start`, which is what `createTestModel()`
uses to pass warnings

## Manual Verification
- Awaiting on the warnings promise no longer causes hanging

Author: danielamitay

.changeset/thin-shoes-switch.md

@@ -0,0 +1,5 @@
+---
+'ai': patch
+---
+
+fix(ai) Make sure warnings promise in streamObject is resolved and properly collects and passes warnings

packages/ai/src/generate-object/stream-object.test.ts

@@ -1798,4 +1798,58 @@ describe('streamObject', () => {
       }
     });
   });
+
+  describe('warnings', () => {
+    it('should resolve warnings promise with undefined when no warnings are present', async () => {
+      const mockModel = createTestModel({
+        warnings: [], // No warnings
+      });
+
+      const result = streamObject({
+        model: mockModel,
+        schema: z.object({ content: z.string() }),
+        prompt: 'prompt',
+      });
+
+      // Consume the stream to completion
+      await convertAsyncIterableToArray(result.partialObjectStream);
+
+      // Wait for the warnings promise to resolve
+      const warnings = await result.warnings;
+
+      expect(warnings).toEqual([]);
+    });
+
+    it('should resolve warnings promise with warnings when warnings are present', async () => {
+      const expectedWarnings: LanguageModelV2CallWarning[] = [
+        {
+          type: 'unsupported-setting',
+          setting: 'frequency_penalty',
+          details: 'This model does not support the frequency_penalty setting.',
+        },
+        {
+          type: 'other',
+          message: 'Test warning message',
+        },
+      ];
+
+      const mockModel = createTestModel({
+        warnings: expectedWarnings,
+      });
+
+      const result = streamObject({
+        model: mockModel,
+        schema: z.object({ content: z.string() }),
+        prompt: 'prompt',
+      });
+
+      // Consume the stream to completion
+      await convertAsyncIterableToArray(result.partialObjectStream);
+
+      // Wait for the warnings promise to resolve
+      const warnings = await result.warnings;
+
+      expect(warnings).toEqual(expectedWarnings);
+    });
+  });
 });

packages/ai/src/generate-object/stream-object.ts

@@ -521,6 +521,7 @@ class DefaultStreamObjectResult<PARTIAL, RESULT, ELEMENT_STREAM>
               case 'response-metadata':
               case 'finish':
               case 'error':
+              case 'stream-start':
                 controller.enqueue(chunk);
                 break;
             }
@@ -715,6 +716,7 @@ class DefaultStreamObjectResult<PARTIAL, RESULT, ELEMENT_STREAM>
                     // resolve promises that can be resolved now:
                     self._usage.resolve(usage);
                     self._providerMetadata.resolve(providerMetadata);
+                    self._warnings.resolve(warnings);
                     self._response.resolve({
                       ...fullResponse,
                       headers: response?.headers,