Workhorse: Increase body limit for DW http requests

What does this MR do and why?

We allow 4mb gRPC messages while we truncate the HTTP response body to 1mb. Since the body is truncated, Duo Workflow Service receives an invalid json and we receive the following class of errors (logs).

Let's increase the limit to 4mb at least to accommodate large checkpoints. The duo-workflow endpoints are now isolated in ai-assisted fleet: https://log.gprd.gitlab.net/app/r/s/JtTXc, so memory consumption has impact only on ai-assisted related requests.

Related to Send only basic checkpoint info on creation (!205536 - merged)