fix(llm): send assistant text as input_text in OpenAI Responses requests (#42613)

[j34ni]

Issue for this PR

Closes #42613

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

When conversation history contains an assistant text message, the OpenAI
Responses (/responses) request body used:

{"role": "assistant", "content": [{"type": "output_text", "text": "..."}]}

output_text is not a valid easy-input content part: per the official OpenAI
schema (openai-python types), assistant easy-input messages accept only
str | input_text | input_image | input_file. Real api.openai.com tolerates
the non-conformant shape, but strict OpenAI-compatible servers validating with
the official SDK types (e.g. a self-hosted Qwen server exposing /v1/responses)
reject it with a 422/400 pydantic ValidationError, so switching a session to
such a model fails.

The fix lowers assistant text content as input_text in request bodies:

• packages/llm/src/protocols/openai-responses.ts:
  • Remove the now-unused OpenAIResponsesOutputText schema.
  • Lower assistant text content as input_text in request bodies
    (lowerMessages).
  • OpenAIResponsesInputItem assistant schema now uses input_text parts.
• packages/llm/test/provider/openai-responses.test.ts: update expected request
  bodies (output_text → input_text for assistant input messages).

The AI SDK path (@ai-sdk/openai convertToOpenAIResponsesMessages) emits the
same invalid shape and would need a separate fix upstream (vercel/ai) to fix
released builds.

How did you verify your code works?

cd packages/llm
bun test test/provider/openai-responses.test.ts

54 pass, 0 fail.

Screenshots / recordings

N/A — not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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