This release introduces a major version change to reflect a breaking modification in the `ResponseFunctionToolCallOutputItem` and `ResponseCustomToolCallOutput` schemas. Specifically, the `output` field, which previously accepted only a `string` value, has been expanded to support multiple structured types:
```
Before: output: string
After: output: string | Array<ResponseInputText | ResponseInputImage | ResponseInputFile>
```
This change allows custom tool calls to return images, files, and rich text content in addition to plain strings, aligning `ResponseCustomToolCallOutput` with the broader `ResponseInput` type system. Because this alters the type and shape of the field, it may break existing callsites that only accept strings.
BREAKING CHANGE: `ResponseFunctionToolCallOutputItem.output` and `ResponseCustomToolCallOutput.output` now return `string | Array<ResponseInputText | ResponseInputImage | ResponseInputFile>` instead of `string` only. This may break existing callsites that assume `output` is always a string.
