Unexpected mapping of AI SDK `system` param with the OpenAI Responses API

Description

When using the AI SDK with the OpenAI Responses API, the AI SDK maps the system param into a system message instead of the instructions field. this is different from the behavior i'd expect. since AI SDK system is so similar to the Responses API instructions, i'd expect the mapping to work.

example:

const result = streamText({
  model: client.responses("some-model"),
  system: "You are a helpful assistant.",
  messages: [{ role: "user", content: "hello, how are you?" }],
});

This is equivalent to:

openai.responses.create({
  model: "some-model"
  input: [
    { role: "system", content: "You are a helpful assistant."},
    { role: "user", content: "hello, how are you?" }],
  instructions: undefined // i'd expect this to be the system message
})

I'd expect the streamText() call to map to:

openai.responses.create({
  model: "some-model"
  input: [
    // only user message here
    { role: "user", content: "hello, how are you?" }],
  instructions: "You are a helpful assistant." // expected
})

AI SDK Version

ai: 5.0.26
@ai-sdk/openai: 5.0.22

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