feat(transport): add non-blocking stdio_pipes transport (#1304)

* feat(transport): add non-blocking stdio_pipes transport

`transport::stdio()` reads and writes through Tokio's blocking thread
pool, so every message on a piped stdio connection pays a cross-thread
hand-off. MCP clients launch stdio servers with FIFOs for stdin/stdout,
which Tokio's Unix pipe driver can drive from the IO reactor instead.

`stdio_pipes()` wraps the standard streams as non-blocking pipes when
both are FIFOs and otherwise falls back to the blocking `stdio()`
handles, so the returned `(StdioReader, StdioWriter)` pair still converts
into a transport on every platform. TTYs, files, sockets, and Windows
keep today's behavior.

`stdio()` is left unchanged because changing its return type would be a
breaking public API change; this constructor is additive.

The `transport-io` feature now enables `tokio/net` for the pipe types.

Refs #1301

* fix(transport): reopen the standard streams on Linux

try_clone_to_owned duplicates the descriptor, so the duplicate shares its open file
description with fd 0 or 1. Tokio's O_NONBLOCK then leaked into the process's own
standard streams, and into children that inherit them.

stdio_pipes() now opens /proc/self/fd/0 and /1 with O_NONBLOCK on Linux. The kernel
gives the new descriptors their own file descriptions, so the flags stay private to the
transport. macOS has no /proc, so it keeps the blocking fallback; the pipe variant is
Linux-only now.

Tests: reopening a FIFO keeps the source descriptor blocking, descriptor 0 keeps its
flags across stdio_pipes(), and the round-trip and non-FIFO cases moved to Linux.

Signed-off-by: itsmunzir <itsmunzir@users.noreply.github.com>

* fix(transport): address review feedback on stdio_pipes

- Reopen fd 0 and 1 first, then check the FIFOs on the descriptors that go
  to Tokio.
- Keep the platform-specific handle in private enums with Debug. Callers
  only need AsyncRead/AsyncWrite.
- Log the taken path with tracing::debug!.
- Drop the vacuous stdin-flags test; the owned-FIFO round trip covers the
  reopen path on the runner.

Signed-off-by: itsmunzir <munzirbh@gmail.com>

* fix(transport): let the FIFO child see EOF in the round-trip test

Signed-off-by: itsmunzir <munzirbh@gmail.com>

---------

Signed-off-by: itsmunzir <itsmunzir@users.noreply.github.com>
Signed-off-by: itsmunzir <munzirbh@gmail.com>
Co-authored-by: itsmunzir <itsmunzir@users.noreply.github.com>

Author: itsmunzir

crates/rmcp/Cargo.toml

@@ -122,6 +122,10 @@ chrono = { version = "0.4.38", default-features = false, features = [
   "oldtime",
 ] }
 
+# the stdio pipe transport reopens `/proc/self/fd` with `O_NONBLOCK`
+[target.'cfg(target_os = "linux")'.dependencies]
+libc = "0.2"
+
 [features]
 default = ["base64", "macros", "server"]
 local = ["rmcp-macros?/local"]
@@ -174,7 +178,7 @@ transport-streamable-http-client-unix-socket = [
 ]
 
 transport-async-rw = ["tokio/io-util", "tokio-util/codec"]
-transport-io = ["transport-async-rw", "tokio/io-std"]
+transport-io = ["transport-async-rw", "tokio/io-std", "tokio/net"]
 transport-child-process = [
   "transport-async-rw",
   "tokio/process",

crates/rmcp/src/transport.rs

@@ -91,7 +91,7 @@ pub use child_process::{ConfigureCommandExt, TokioChildProcess};
 #[cfg(feature = "transport-io")]
 pub mod io;
 #[cfg(feature = "transport-io")]
-pub use io::stdio;
+pub use io::{StdioReader, StdioWriter, stdio, stdio_pipes};
 
 #[cfg(feature = "auth")]
 pub mod auth;