Anthropic should backoff for http 429 responses

[michaelneale]

It's not uncommon to get this:

httpx.HTTPStatusError: Client error '429 Too Many Requests' for url 'https://api.anthropic.com/v1/messages'
For more information check: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/429
{"type":"error","error":{"type":"rate_limit_error","message":"Number of request tokens has exceeded your per-minute rate limit 
(https://docs.anthropic.com/en/api/rate-limits); see the response headers for current usage. Please reduce the prompt length or the maximum tokens requested, or
try again later. You may also contact sales at https://www.anthropic.com/contact-sales to discuss your options for a rate limit increase."}}

  File "/Users/micn/Documents/code/goose/.venv/lib/python3.12/site-packages/exchange/providers/anthropic.py", line 158, in _post
    return raise_for_status(response).json()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/micn/Documents/code/goose/.venv/lib/python3.12/site-packages/exchange/providers/utils.py", line 35, in raise_for_status
    raise httpx.HTTPStatusError(f"{e}\n{response.text}", request=e.request, response=e.response)

Ideally would do a sensible backoff for this or slow down when running