fix(xai): add support for response.function_call_arguments streaming events

[ctate]

When using xAI Grok models (e.g. grok-4-1-fast-reasoning) with function tools via the Responses API, the model streams response.function_call_arguments.delta and response.function_call_arguments.done events — standard Responses API events also used by OpenAI. The @ai-sdk/xai provider's Zod schema did not include these event types, causing AI_TypeValidationError and breaking the entire stream whenever a function tool call was attempted.

Summary

• Added response.function_call_arguments.delta and response.function_call_arguments.done to xaiResponsesChunkSchema in xai-responses-api.ts
• Updated the stream handler in xai-responses-language-model.ts to:
  • Track ongoing tool calls by output_index
  • Emit tool-input-start on response.output_item.added for function_call items
  • Emit tool-input-delta for each response.function_call_arguments.delta event
  • Emit tool-input-end and tool-call on response.output_item.done (once arguments are fully received)
  • Skip response.custom_tool_call_input.delta/done events for function_call items (these are handled by the output_item events)
• Added unit test for function call argument streaming

Manual Verification

Built the patched @ai-sdk/xai locally and ran an e2e test against the real xAI API using grok-4-1-fast-reasoning with a function tool:

Before fix — AI_TypeValidationError on response.function_call_arguments.done:

AI_TypeValidationError: Type validation failed: Value: {
  "sequence_number":4,
  "type":"response.function_call_arguments.done",
  "arguments":"{}",
  "item_id":"fc_14f28817-...",
  "output_index":0
}

After fix — full streaming pipeline works:

tool-input-start:  PASS
tool-input-delta:  PASS
tool-call:         PASS
no errors:         PASS

Checklist

• Tests have been added / updated (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• A patch changeset for relevant packages has been added (for bug fixes / features - run pnpm changeset in the project root)
• I have reviewed this pull request (self-review)

Related Issues

Fixes function tool calls failing with AI_TypeValidationError for xAI Grok models using the Responses API.