# [Bug]: Proxy forwards internal `_litellm_*` reservation fields to OpenAI chat completions

## Description

LiteLLM proxy can attach internal rate-limiter / TPM reservation bookkeeping fields directly to the request payload dict, then forward the same dict to the upstream OpenAI Chat Completions API.

When that happens, OpenAI rejects the request with errors like:

- `Unknown parameter: '_litellm_rate_limit_descriptors'.` - `Unrecognized request arguments supplied: _litellm_rate_limit_descriptors, _litellm_tpm_reserved_model, _litellm_tpm_reserved_scopes, _litellm_tpm_reserved_tokens`

In our environment this showed up from LiteLLM proxy as `RateLimitError` / `429`, but the upstream OpenAI response was actually a `400 invalid_request_error`.

## Observed behavior

Proxy-side logs showed failures on `POST /v1/chat/completions` with messages equivalent to:

```text OpenAIException - Unrecognized request arguments supplied: _litellm_rate_limit_descriptors, _litellm_tpm_reserved_model, _litellm_tpm_reserved_scopes, _litellm_tpm_reserved_tokens ```

and:

```text OpenAIException - Unknown parameter: '_litellm_rate_limit_descriptors' ```

The failures were reproducible on OpenAI-backed …