fix: remove unused CommandExt import in unix cfg blocks

tokio::process::Command already exposes process_group() natively on
Unix — the std::os::unix::process::CommandExt trait import is redundant.
Clippy on Linux CI catches this as an unused import (promoted to error
with -D warnings). On Windows locally, #[cfg(unix)] blocks are compiled
out entirely so clippy never sees them.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Mohammod Al Amin Ashik <maa.ashik00@gmail.com>

Author: its-mash

crates/mcpmux-gateway/src/pool/transport/stdio.rs

@@ -37,7 +37,6 @@ pub fn configure_child_process_platform(cmd: &mut Command) {
     }
     #[cfg(unix)]
     {
-        use std::os::unix::process::CommandExt;
         cmd.process_group(0);
     }
 }

crates/mcpmux-mcp/src/transports.rs

@@ -162,7 +162,6 @@ impl McpSession {
                 }
                 #[cfg(unix)]
                 {
-                    use std::os::unix::process::CommandExt;
                     cmd.process_group(0);
                 }
             })