429 responses w/o retry_after_ms cause exception

Sometimes (no idea how this is triggered) synapse sends responses with a 429 status code without the `retry_after_ms` field. In those cases, the python client crashes with a `KeyError` since https://github.com/matrix-org/matrix-python-sdk/blob/401439f020ba79197c719a9b2d51838d833b731e/matrix_client/api.py#L666 unconditionally expects `retry_after_ms` to be available on 429 responses.

A relatively simple solution would be to have our own internal exponential backoff counter that gets reset every time a request succeeds or an explicit `retry_after_ms` value is received.

What do you think?

Start at matrix_client/api.py around line 666 and inspect the 429 response handling. Done means a 429 response without retry_after_ms no longer raises KeyError, while the existing explicit retry_after_ms behavior and the proposed backoff behavior are addressed.

Written by the indexing model from the issue text.