refactor(transport): move which_command behind opt-in feature flag

Address review feedback: the `which` dependency is now gated behind a
separate `which-command` feature flag instead of being bundled into
`transport-child-process`. Users on Linux/macOS who don't need
cross-platform executable resolution no longer pull in the extra crate.

Also fixes the doc example import path to use the re-exported
`rmcp::transport::which_command`.

Author: kakarot-dev

crates/rmcp/Cargo.toml

@@ -136,6 +136,9 @@ transport-child-process = [
   "transport-async-rw",
   "tokio/process",
   "dep:process-wrap",
+]
+which-command = [
+  "transport-child-process",
   "dep:which",
 ]
 transport-streamable-http-server = [

crates/rmcp/src/transport.rs

@@ -83,8 +83,10 @@ pub use worker::WorkerTransport;
 
 #[cfg(feature = "transport-child-process")]
 pub mod child_process;
+#[cfg(feature = "which-command")]
+pub use child_process::which_command;
 #[cfg(feature = "transport-child-process")]
-pub use child_process::{ConfigureCommandExt, TokioChildProcess, which_command};
+pub use child_process::{ConfigureCommandExt, TokioChildProcess};
 
 #[cfg(feature = "transport-io")]
 pub mod io;

crates/rmcp/src/transport/child_process.rs

@@ -242,7 +242,7 @@ impl ConfigureCommandExt for tokio::process::Command {
 ///
 /// # Example
 /// ```rust,no_run
-/// use rmcp::transport::child_process::{which_command, ConfigureCommandExt};
+/// use rmcp::transport::{which_command, ConfigureCommandExt};
 ///
 /// # fn example() -> std::io::Result<()> {
 /// let cmd = which_command("npx")?
@@ -252,6 +252,7 @@ impl ConfigureCommandExt for tokio::process::Command {
 /// # Ok(())
 /// # }
 /// ```
+#[cfg(feature = "which-command")]
 pub fn which_command(
     name: impl AsRef<std::ffi::OsStr>,
 ) -> std::io::Result<tokio::process::Command> {
@@ -260,6 +261,7 @@ pub fn which_command(
     Ok(tokio::process::Command::new(resolved))
 }
 
+#[cfg(feature = "which-command")]
 #[cfg(test)]
 mod tests_which {
     #[test]