stdio transport: a request over the maximum line length gets no answer #1395

Since #1049 (rmcp 3.6.0) AsyncRwTransport drops a line that is longer than the maximum line length (16 MiB by default). It only writes a log entry. When that line is a request, the peer never gets an answer and waits until its own timeout. From the client side this looks like a server that hangs.

I found this while writing an MCP server that takes large inline files. A tools/call with a large base64 file in the arguments is enough.

To reproduce:

1. Start any stdio server built on rmcp 3.7.0.
2. Send initialize and notifications/initialized.
3. Send one tools/call request whose line is larger than 16 MiB, for example with a 17 MiB string in arguments.
4. The server logs "Incoming message exceeded the maximum line length, discarding it". The client gets nothing for this id.

Expected: an error answer for the request id, like the -32600 Invalid Request answer rmcp already sends for valid JSON with the wrong shape.

Lines without an id, responses and input that is not JSON should stay unanswered, so this does not bring back the error storm from #938.

I have a fix ready that builds on #1370. I can open it after that PR is merged.