feat(transport): add which_command for cross-platform executable resolution (#774)

* feat(transport): add which_command for cross-platform executable resolution

Adds a `which_command()` helper that resolves executable paths via the
`which` crate before constructing a `tokio::process::Command`. This fixes
Windows failures where `.cmd` shim scripts (e.g. `npx.cmd`) are not
found by `Command::new()` without a fully-qualified path.

Closes #456

* 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`.

crates/rmcp/Cargo.toml

@@ -81,6 +81,9 @@ tower-service = { version = "0.3", optional = true }
 # for child process transport
 process-wrap = { version = "9.0", features = ["tokio1"], optional = true }
 
+# for cross-platform executable path resolution
+which = { version = "7", optional = true }
+
 # for ws transport
 # tokio-tungstenite ={ version = "0.26", optional = true }
 
@@ -163,6 +166,10 @@ transport-child-process = [
   "tokio/process",
   "dep:process-wrap",
 ]
+which-command = [
+  "transport-child-process",
+  "dep:which",
+]
 transport-streamable-http-server = [
   "transport-streamable-http-server-session",
   "server-side-http",