## Bug

`previous_models` in the Router's request metadata has two issues:

### 1. Cross-request data leakage

`self.previous_models` is stored on the Router instance (router.py:544), not per-request. When request A fails and request B fails next, request B's `previous_models` contains request A's full metadata — including a different user's `user_api_key_auth`, `end_user_id`, tools, and `extra_headers` (which may contain OAuth bearer tokens).

Example: a request from user `UVk9MB0JsREqR7tH1liddPw=` with 10 tools had `previous_models` containing a completely different request from user `U2UlBAuuRG0q_faub5t4xyA=` with 19 tools and 3.2MB of base64 image data.

### 2. Massive metadata bloat

`log_retry` (router.py:5850) copies almost everything from `kwargs` into each `previous_models` entry — tools, full metadata dict, user_api_key_auth dumps, extra_headers with auth tokens, etc. With 3 retries and large tool schemas, this adds megabytes of redundant data to the spend log for each failed request.

Each failed attempt already has its own spend log entry with full error details, so duplicating all this data in `previous_models` is redundant.

## Suggested Fix

Replace the full kwargs d…