### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

When a request to LiteLLM's `/responses` proxy endpoint contains a `function_call_output` whose `output` array includes an `input_file` content part (e.g. a PDF returned from a tool call, as produced by the OpenAI Agents SDK's `ToolOutputFileContent`), the Responses→ChatCompletions transformer **silently drops the `input_file` part**. Only the sibling `input_text` parts survive into the resulting `role: "tool"` message's `content`, flattened to a plain string.

This affects every provider that lacks a native `/responses` endpoint and therefore goes through the lowering path. **Confirmed on `vertex_ai/gemini-2.5-flash`**; very likely the same on `bedrock/*` (same lowering code path). **Azure is not affected** because LiteLLM passes through to Azure OpenAI's native `/openai/responses` endpoint and the original Responses-API body — including the `input_file` part — reaches the provider intact.

**Expected:** the file content is forwarded to the provider (as a chat-completions `{"type": "file", "file": {...}}` part, analogous to how `input_image` i…