feat(cli): show ▶ N indicator in status bar when /background tasks are running (#27175)

Surface live background-task count in the prompt_toolkit status bar so users
can see at a glance that a /background task exists and is running — no need
to ask the agent about it (the agent has no visibility into bg sessions by
design).

- _get_status_bar_snapshot now reports active_background_tasks from len()
  of the live _background_tasks dict (entries are removed in the task
  thread's finally block, so this reflects truly-running tasks)
- Indicator shown only on medium (<76) and wide (>=76) tiers; narrow (<52)
  stays minimal since it's already cramped
- No invalidate plumbing needed: status bar fragments are pulled via lambda
  on every redraw, and the bg thread already calls _app.invalidate() on exit

Refs #8568

tests/cli/test_cli_background_status_indicator.py

@@ -0,0 +1,104 @@
+"""Tests for the /background indicator in the CLI status bar.
+
+The classic prompt_toolkit status bar shows `▶ N` when N tasks launched via
+`/background` are still running. Source of truth is `self._background_tasks`
+(a Dict[str, threading.Thread]); entries are removed in the task thread's
+finally block, so len() reflects truly-running tasks.
+"""
+
+import threading
+from datetime import datetime
+
+from cli import HermesCLI
+
+
+def _stub_thread() -> threading.Thread:
+    """Return a Thread instance that's never started — pure dict-value stand-in."""
+    return threading.Thread(target=lambda: None)
+
+
+def _make_cli():
+    """Bare-metal HermesCLI for snapshot/build tests (no __init__ side effects)."""
+    cli_obj = HermesCLI.__new__(HermesCLI)
+    cli_obj.model = "anthropic/claude-opus-4.6"
+    cli_obj.agent = None
+    cli_obj._background_tasks = {}
+    # The snapshot reads session_start to compute duration; supply a stub.
+    cli_obj.session_start = datetime.now()
+    return cli_obj
+
+
+def test_snapshot_reports_zero_when_no_background_tasks():
+    cli_obj = _make_cli()
+    snap = cli_obj._get_status_bar_snapshot()
+    assert snap["active_background_tasks"] == 0
+
+
+def test_snapshot_counts_live_background_tasks():
+    cli_obj = _make_cli()
+    cli_obj._background_tasks = {"bg_a": _stub_thread(), "bg_b": _stub_thread()}
+    snap = cli_obj._get_status_bar_snapshot()
+    assert snap["active_background_tasks"] == 2
+
+
+def test_snapshot_safe_when_background_tasks_attr_missing():
+    """Older HermesCLI instances (tests with __new__, etc.) may lack the attr."""
+    cli_obj = HermesCLI.__new__(HermesCLI)
+    cli_obj.model = "x"
+    cli_obj.agent = None
+    cli_obj.session_start = datetime.now()
+    # No _background_tasks at all — must not raise.
+    snap = cli_obj._get_status_bar_snapshot()
+    assert snap["active_background_tasks"] == 0
+
+
+def test_plain_text_status_omits_indicator_when_idle():
+    cli_obj = _make_cli()
+    text = cli_obj._build_status_bar_text(width=80)
+    assert "▶" not in text
+
+
+def test_plain_text_status_shows_indicator_when_active():
+    cli_obj = _make_cli()
+    cli_obj._background_tasks = {"bg_a": _stub_thread()}
+    text = cli_obj._build_status_bar_text(width=80)
+    assert "▶ 1" in text
+
+
+def test_plain_text_status_shows_higher_count():
+    cli_obj = _make_cli()
+    cli_obj._background_tasks = {
+        "a": _stub_thread(),
+        "b": _stub_thread(),
+        "c": _stub_thread(),
+    }
+    text = cli_obj._build_status_bar_text(width=80)
+    assert "▶ 3" in text
+
+
+def test_narrow_width_omits_bg_indicator():
+    """The narrow tier (<52) is already cramped — bg is secondary, drop it."""
+    cli_obj = _make_cli()
+    cli_obj._background_tasks = {"bg_a": _stub_thread()}
+    text = cli_obj._build_status_bar_text(width=40)
+    assert "▶" not in text
+
+
+def test_fragments_include_bg_segment_when_active():
+    cli_obj = _make_cli()
+    cli_obj._background_tasks = {"a": _stub_thread(), "b": _stub_thread()}
+    cli_obj._status_bar_visible = True
+    # _get_status_bar_fragments asks _get_tui_terminal_width(); stub it wide.
+    cli_obj._get_tui_terminal_width = lambda: 120  # type: ignore[method-assign]
+    frags = cli_obj._get_status_bar_fragments()
+    rendered = "".join(text for _style, text in frags)
+    assert "▶ 2" in rendered
+
+
+def test_fragments_omit_bg_segment_when_idle():
+    cli_obj = _make_cli()
+    cli_obj._status_bar_visible = True
+    cli_obj._get_tui_terminal_width = lambda: 120  # type: ignore[method-assign]
+    frags = cli_obj._get_status_bar_fragments()
+    rendered = "".join(text for _style, text in frags)
+    assert "▶" not in rendered