Retry with backoff on 429 and 5xx responses

[TheCoder30ec4]

Motivation

Jev and OpenRouter can rate-limit (HTTP 429) or fail briefly (502/503). Right now one hiccup fails route() straight away.

Proposal

• Retry 429, 500, 502, 503 and 504 up to N times (default 2) with exponential backoff plus jitter
• Honor the Retry-After header when present
• Never retry other 4xx codes
• Make it configurable: Router(..., max_retries=2)

Stdlib only (time.sleep). No new dependencies.

Where

src/model_router/_http.py, plus passing the setting through from Router.

Done when

• Retries are tested with a mocked urlopen and a mocked sleep, so tests stay fast
• README documents the default