Unbounded response body buffering in Streamable HTTP clients

Summary

Before 3.9.0, the built-in Streamable HTTP clients in rmcp read non-streaming HTTP response bodies into memory with no size limit. Both the reqwest client (`transport-streamable-http-client-reqwest`) and the Unix socket client (`transport-streamable-http-client-unix-socket`) were affected:

- `application/json` responses to POST requests were read in full before parsing. - Non-2xx responses were read in full for error reporting.

A malicious or compromised MCP server can answer with a body that never ends, for example a chunked `application/json` response that streams whitespace. The client buffers it until the process runs out of memory. This works during `initialize`, before any authentication. On desktop or agent hosts, this terminates the whole host application.

Server-sent event streams are not affected. Each event is already capped by `max_sse_event_size`, and events are not accumulated.

The flaw was corrected in 3.9.0. Successful JSON bodies are limited to 16 MiB and non-success bodies to 64 KiB. Bytes are counted while streaming, after decompression. An oversized `Content-Length` is rejected before the body is read, and an oversized body is rejected with `StreamableHttpError::ResponseBodyTooLarge` instead of being truncated.

If you cannot upgrade, use a custom `StreamableHttpClient` that enforces a response body limit, or connect only to trusted MCP servers.