Fix pipe redirection into complete (#12818)

# Description
Fixes #12796 where a combined out and err pipe redirection (`o+e>|`)
into `complete` still provides separate `stdout` and `stderr` columns in
the record. Now, the combined output will be in the `stdout` column.
This PR also fixes a similar error with the `e>|` pipe redirection.

# Tests + Formatting
Added two tests.

crates/nu-command/tests/commands/complete.rs

@@ -92,3 +92,16 @@ fn capture_error_with_both_stdout_stderr_messages_not_hang_nushell() {
         },
     )
 }
+
+#[test]
+fn combined_pipe_redirection() {
+    let actual = nu!("$env.FOO = hello; $env.BAR = world; nu --testbin echo_env_mixed out-err FOO BAR o+e>| complete | get stdout");
+    assert_eq!(actual.out, "helloworld");
+}
+
+#[test]
+fn err_pipe_redirection() {
+    let actual =
+        nu!("$env.FOO = hello; nu --testbin echo_env_stderr FOO e>| complete | get stdout");
+    assert_eq!(actual.out, "hello");
+}

crates/nu-engine/src/eval.rs

@@ -340,7 +340,13 @@ fn eval_redirection<D: DebugContext>(
             }
             Ok(Redirection::file(options.create(true).open(path)?))
         }
-        RedirectionTarget::Pipe { .. } => Ok(Redirection::Pipe(next_out.unwrap_or(OutDest::Pipe))),
+        RedirectionTarget::Pipe { .. } => {
+            let dest = match next_out {
+                None | Some(OutDest::Capture) => OutDest::Pipe,
+                Some(next) => next,
+            };
+            Ok(Redirection::Pipe(dest))
+        }
     }
 }
 
@@ -367,11 +373,12 @@ fn eval_element_redirection<D: DebugContext>(
             } => {
                 let stderr = eval_redirection::<D>(engine_state, stack, target, None)?;
                 if matches!(stderr, Redirection::Pipe(OutDest::Pipe)) {
+                    let dest = match next_out {
+                        None | Some(OutDest::Capture) => OutDest::Pipe,
+                        Some(next) => next,
+                    };
                     // e>| redirection, don't override current stack `stdout`
-                    Ok((
+                    Ok((None, Some(Redirection::Pipe(dest))))
-                        None,
-                        Some(next_out.map(Redirection::Pipe).unwrap_or(stderr)),
-                    ))
                 } else {
                     Ok((next_out.map(Redirection::Pipe), Some(stderr)))
                 }