acp_thread: Fix terminal tool incorrectly redirecting stdin to /dev/null (#39092)

Closes https://github.com/zed-industries/zed/issues/38462

Release Notes:

- Fixed AI terminal tool incorrectly redirecting stdin to `/dev/null`

crates/task/src/shell_builder.rs

@@ -210,6 +210,7 @@ pub struct ShellBuilder {
     program: String,
     args: Vec<String>,
     interactive: bool,
+    /// Whether to redirect stdin to /dev/null for the spawned command as a subshell.
     redirect_stdin: bool,
     kind: ShellKind,
 }
@@ -283,10 +284,12 @@ impl ShellBuilder {
             if self.redirect_stdin {
                 match self.kind {
                     ShellKind::Posix | ShellKind::Nushell | ShellKind::Fish | ShellKind::Csh => {
-                        combined_command.push_str(" </dev/null");
+                        combined_command.insert(0, '(');
+                        combined_command.push_str(") </dev/null");
                     }
                     ShellKind::PowerShell => {
-                        combined_command.insert_str(0, "$null | ");
+                        combined_command.insert_str(0, "$null | {");
+                        combined_command.push_str("}");
                     }
                     ShellKind::Cmd => {
                         combined_command.push_str("< NUL");
@@ -333,4 +336,17 @@ mod test {
             ]
         );
     }
+
+    #[test]
+    fn redirect_stdin_to_dev_null_precedence() {
+        let shell = Shell::Program("nu".to_owned());
+        let shell_builder = ShellBuilder::new(None, &shell);
+
+        let (program, args) = shell_builder
+            .redirect_stdin_to_dev_null()
+            .build(Some("echo".into()), &["nothing".to_string()]);
+
+        assert_eq!(program, "nu");
+        assert_eq!(args, vec!["-i", "-c", "(echo nothing) </dev/null"]);
+    }
 }