Graceful handling of chat credit exhaustion

This release surfaces a clear message when an organization runs out of chat credits and exposes utilities so downstream consumers can react to the same condition.

Features

• Credits-exhausted message and stream error exports #2921  - When an organization runs out of chat credits, the thread now renders a clear credit-limit message instead of silently stopping the stream on a 402 from the gateway. New describeStreamError and CREDITS_EXHAUSTED_MESSAGE exports let downstream consumers detect and react to the same condition. (Author: @simplesagar )