Fix multimodal tool-result translation in Messages → Responses

Problem

Claude Code screenshots arrive as Anthropic image blocks nested inside tool_result.content.

Floway currently passes all tool results through flattenMessagesToolResult(). Any array containing an image is JSON-serialized into a string, including the full base64 payload.

The translated Responses request therefore contains:

{
  "type": "function_call_output",
  "call_id": "toolu_123",
  "output": "[{\"type\":\"image\",\"source\":{\"type\":\"base64\",\"media_type\":\"image/png\",\"data\":\"...\"}}]"
}

The image becomes prompt text instead of a native Responses image. This bypasses Floway's existing image-compression and vision-header interceptors and can cause screenshot-heavy or compacted Claude Code requests to exceed the upstream prompt limit.

Reproduction

Send a Messages request through a Responses-backed model with an image inside a tool result:

{
  "role": "user",
  "content": [
    {
      "type": "tool_result",
      "tool_use_id": "toolu_123",
      "content": [
        {
          "type": "text",
          "text": "before"
        },
        {
          "type": "image",
          "source": {
            "type": "base64",
            "media_type": "image/png",
            "data": "<base64>"
          }
        },
        {
          "type": "text",
          "text": "after"
        }
      ]
    }
  ]
}

Actual behavior

function_call_output.output is a JSON string containing the image base64.

Expected behavior

Image-bearing tool results should remain native multimodal Responses output:

{
  "type": "function_call_output",
  "call_id": "toolu_123",
  "output": [
    {
      "type": "input_text",
      "text": "before"
    },
    {
      "type": "input_image",
      "image_url": "data:image/png;base64,<base64>"
    },
    {
      "type": "input_text",
      "text": "after"
    }
  ],
  "status": "completed"
}

Existing behavior should remain unchanged for string, empty, text-only, and search-result-only tool results.

Proposed fix

Update:

packages/translate/src/messages-via-responses/request.ts

1. Extract and reuse the existing Messages image → Responses input_image conversion.
2. Add a Responses-specific tool-output projection:
  - If no image is present, continue using flattenMessagesToolResult().
  - If an image is present, preserve source order:
      - text → input_text
    - image → input_image
    - search_result → input_text containing its existing JSON representation

- packages/translate/src/shared/messages-via/tool-result.ts
- Messages → Chat Completions translation
- Protocol declarations
- Provider interceptors

Chat Completions requires string-valued tool messages, while Responses already permits array-valued multimodal tool output.

Regression coverage

Add tests in:

packages/translate/__tests__/messages-via-responses/request_test.ts

Cover:

- String tool results retain string output
- Empty arrays retain empty-string output
- Text-only arrays retain existing flattening
- Search-result-only arrays retain existing JSON output
- text/image/text preserves order
- Image data URLs are exact
- Image base64 is not embedded in a JSON string
- is_error: true preserves status: "incomplete"
- Mixed image/search-result content preserves order
- Surrounding content remains message → function_call_output → message

Local validation

A local implementation has been tested successfully:

- Messages → Responses request tests: 45/45 passed
- Responses compression and vision-header tests: 14/14 passed
- Full test suite: 5,433/5,433 passed
- Typecheck: passed
- ESLint: passed
- Live /v1/messages request with multiple screenshot tool results: passed
- Compacted-history-style screenshot request: passed

The patched local proxy correctly processes array-valued function_call_output.output containing native input_image parts.

Start in packages/translate/src/messages-via-responses/request.ts and inspect the existing Messages image-to-Responses conversion, then read packages/translate/src/shared/messages-via/tool-result.ts. Run packages/translate/tests/messages-via-responses/request_test.ts first. Done means image-bearing tool results preserve source order as native output parts, while existing string, empty, text-only, and search-result behavior and error status remain covered by passing tests.