fix(openai): handle file-url parts in Responses tool result content

[s-zx]

Summary

The OpenAI Responses provider converts tool result content parts into API input items inside convertToOpenAIResponsesInput. Three content part types were already handled:

Part type	Mapped to
text	input_text
image-data	input_image (base64)
image-url	input_image (url)
file-data	input_file (base64)

However file-url parts fell through to the default branch, which emits a warning and returns undefined — causing the part to be silently dropped.

Root cause

In packages/openai/src/responses/convert-to-openai-responses-input.ts, both the function_call_output content mapping and the custom_tool_call_output content mapping were missing a file-url case.

Fix

Add a file-url case in both switch blocks that maps to { type: 'input_file', file_url: item.url }, consistent with how the OpenAI Responses API represents file URLs:

case 'file-url': {
  return {
    type: 'input_file' as const,
    file_url: item.url,
  };
}

This mirrors the existing file-data path and matches the input_file shape already used for PDF URL parts in the user message handler.

Fixes #13622

[vercel]

Additional Suggestion:

The OpenAIResponsesFunctionCallOutput type's output array union is missing the { type: 'input_file'; file_url: string } variant, causing a type mismatch with code that produces this variant.