Unbounded tool schema cache in Streamable HTTP server transport

Summary

From 3.0.0 until 3.6.0, `StreamableHttpService` cached the result of `ServerHandler::get_tool` in a process-wide map keyed by the tool name of incoming `tools/call` requests. The map had no size limit and no eviction, and it also cached unknown tool names.

An entry was added while the request headers were validated, before the request could be rejected, so requests answered with HTTP 400 still grew the map. A client that can reach the endpoint can send `tools/call` requests with distinct tool names, each up to the default 4 MiB body limit, until the server runs out of memory. rmcp does not authenticate requests itself, so servers without an authentication layer in front of the endpoint are exposed to unauthenticated clients.

Only servers that use the Streamable HTTP server transport and receive requests at protocol version `2026-07-28` or later are affected.

The flaw was corrected in 3.6.0 by removing the cache. The transport now calls `ServerHandler::get_tool` on each request.

If you cannot upgrade, put authentication in front of the endpoint, or reject `tools/call` requests for unknown tool names before they reach `StreamableHttpService`.