Remove signal termination handling per feedback

Remove code >= 128 signal handling from all 3 locations:
- handle_exec_result! macro
- insert mode handler
- final args handler

Keep INSERT_ARG_MAX constant and validation as requested.
All 25 tests still passing.

Co-authored-by: jgarzik <494411+jgarzik@users.noreply.github.com>

Author: Copilot

process/xargs.rs

@@ -568,11 +568,6 @@ macro_rules! handle_exec_result {
             ExecResult::Exited(255) => {
                 return Ok(SpawnResult { exit_code: 1 });
             }
-            ExecResult::Exited(code) if code >= 128 => {
-                // POSIX: utility terminated by signal - exit immediately
-                eprintln!("xargs: command terminated by signal");
-                return Ok(SpawnResult { exit_code: 1 });
-            }
             ExecResult::Exited(code) if code != 0 => {
                 $any_failed = true;
             }
@@ -703,11 +698,6 @@ fn read_and_spawn(args: &Args) -> io::Result<SpawnResult> {
                 ExecResult::Exited(255) => {
                     return Ok(SpawnResult { exit_code: 1 });
                 }
-                ExecResult::Exited(code) if code >= 128 => {
-                    // POSIX: utility terminated by signal - exit immediately
-                    eprintln!("xargs: command terminated by signal");
-                    return Ok(SpawnResult { exit_code: 1 });
-                }
                 ExecResult::Exited(code) if code != 0 => {
                     any_failed = true;
                 }
@@ -729,11 +719,6 @@ fn read_and_spawn(args: &Args) -> io::Result<SpawnResult> {
             ExecResult::Exited(255) => {
                 return Ok(SpawnResult { exit_code: 1 });
             }
-            ExecResult::Exited(code) if code >= 128 => {
-                // POSIX: utility terminated by signal - exit immediately
-                eprintln!("xargs: command terminated by signal");
-                return Ok(SpawnResult { exit_code: 1 });
-            }
             ExecResult::Exited(code) if code != 0 => {
                 any_failed = true;
             }