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

[kakarot-dev]

Summary

Adds a which_command() helper function 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.

Changes:

• Added which_command(name) -> io::Result<Command> to child_process.rs
• Added which v7 as an optional dependency, gated behind transport-child-process feature
• Re-exported which_command from transport module
• Added tests for known binary resolution and nonexistent binary error handling

Usage:

use rmcp::transport::{which_command, ConfigureCommandExt};

let cmd = which_command("npx")?
    .configure(|cmd| {
        cmd.arg("-y").arg("@modelcontextprotocol/server-everything");
    });

Closes #456