[aisdk] Allow for empty reasoning blocks (observed behavior) (#580)

## Summary
Sometimes the model does return an empty reasoning block... Let's let it
through instead of failing to decode.

Example:

https://platform.openai.com/logs/resp_02474882b463981300691523a7eb9481938b7823951cfe62ab

I'll add the raw response in a comment to this PR

## How was it tested?
Ran testpilot locally.

## Community Contribution License

All community contributions in this pull request are licensed to the
project maintainers under the terms of the [Apache 2
License](https://www.apache.org/licenses/LICENSE-2.0).

By creating this pull request I represent that I have the right to
license the contributions to the project maintainers under the Apache 2
License as stated in the [Community Contribution
License](https://github.com/jetify-com/opensource/blob/main/CONTRIBUTING.md#community-contribution-license).

Author: ipince

aisdk/ai/provider/openai/internal/codec/decode.go

@@ -110,9 +110,6 @@ func decodeReasoning(item responses.ResponseOutputItemUnion) (*api.ReasoningBloc
 	}
 
 	reasoningItem := item.AsReasoning()
-	if len(reasoningItem.Summary) == 0 {
-		return nil, fmt.Errorf("reasoning item has no summary")
-	}
 
 	// For now, we'll concatenate all summary texts with newlines if there are multiple
 	// Another option would be to return a slice of ReasoningBlocks.
@@ -125,9 +122,9 @@ func decodeReasoning(item responses.ResponseOutputItemUnion) (*api.ReasoningBloc
 		texts = append(texts, summary.Text)
 	}
 
-	if len(texts) == 0 {
-		return nil, fmt.Errorf("no valid text found in reasoning summaries")
-	}
+	// NOTE: we don't check that the text is non-empty because sometimes the model returns an empty
+	// reasoning block. That behavior is technically against the API spec, but it's been observed in practice.
+	// Consider omitting the ReasoningBlock and adding a warning instead.
 
 	return &api.ReasoningBlock{
 		Text: strings.Join(texts, "\n"),

aisdk/ai/provider/openai/internal/codec/decode_test.go

@@ -1192,7 +1192,7 @@ func TestDecodeReasoning(t *testing.T) {
 				"id": "reason_123",
 				"summary": []
 			}`,
-			wantErr: "reasoning item has no summary",
+			want: &api.ReasoningBlock{},
 		},
 		{
 			name: "empty text in summary",