Every image in history is base64-inlined into every request, so a long
session's request body grows until a gateway request-size cap rejects it
with 413 and every retry resends the same oversized body, permanently
wedging the session. Each provider route now carries maxRequestImageBytes
(default 24MiB): when the accumulated base64 image payload exceeds it,
the oldest images are replaced by a fixed model-facing placeholder until
the request fits, so the newest images survive and the session keeps
completing requests. 413 and request-body-cap wording now classify as
INVALID_REQUEST instead of the generic PI_AI_ERROR.
Fixes#2644
creatixchu
0b4a322003