Show placeholder for commands with no output (#4509)

## Summary
- show a dim “(no output)” placeholder when an executed command produces
no stdout or stderr so empty runs are visible
- update TUI snapshots to include the new placeholder in history
renderings

## Testing
- cargo test -p codex-tui


------
https://chatgpt.com/codex/tasks/task_i_68dc056c1d5883218fe8d9929e9b1657

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__interrupt_exec_marks_failed.snap

@@ -3,3 +3,4 @@ source: tui/src/chatwidget/tests.rs
 expression: exec_blob
 ---
 • Ran sleep 1
+  └ (no output)

codex-rs/tui/src/exec_cell/render.rs

@@ -366,6 +366,14 @@ impl ExecCell {
                     include_prefix: false,
                 },
             );
+
+            if raw_output_lines.is_empty() {
+                lines.extend(prefix_lines(
+                    vec![Line::from("(no output)".dim())],
+                    Span::from(layout.output_block.initial_prefix).dim(),
+                    Span::from(layout.output_block.subsequent_prefix),
+                ));
+            } else {
                 let trimmed_output =
                     Self::truncate_lines_middle(&raw_output_lines, layout.output_max_lines);
 
@@ -388,6 +396,7 @@ impl ExecCell {
                     ));
                 }
             }
+        }
 
         lines
     }

codex-rs/tui/src/snapshots/codex_tui__history_cell__tests__multiline_command_both_lines_wrap_with_correct_prefixes.snap

@@ -6,3 +6,4 @@ expression: rendered
   │ ough_to_wrap
   │ second_token_is_also_lon
   │ … +1 lines
+  └ (no output)

codex-rs/tui/src/snapshots/codex_tui__history_cell__tests__multiline_command_without_wrap_uses_branch_then_eight_spaces.snap

@@ -4,3 +4,4 @@ expression: rendered
 ---
 • Ran echo one
   │ echo two
+  └ (no output)

codex-rs/tui/src/snapshots/codex_tui__history_cell__tests__multiline_command_wraps_with_extra_indent_on_subsequent_lines.snap

@@ -5,3 +5,4 @@ expression: rendered
 • Ran set -o pipefail
   │ cargo test
   │ --all-features --quiet
+  └ (no output)

codex-rs/tui/src/snapshots/codex_tui__history_cell__tests__single_line_command_compact_when_fits.snap

@@ -3,3 +3,4 @@ source: tui/src/history_cell.rs
 expression: rendered
 ---
 • Ran echo ok
+  └ (no output)

codex-rs/tui/src/snapshots/codex_tui__history_cell__tests__single_line_command_wraps_with_four_space_continuation.snap

@@ -5,3 +5,4 @@ expression: rendered
 • Ran a_very_long_token_
   │ without_spaces_to_
   │ force_wrapping
+  └ (no output)