fix(approval): close remaining prompt_toolkit deadlock vectors (#15216)

PR #13734 fixed the concurrent-tool-executor vector (ThreadPoolExecutor
workers didn't inherit the CLI's TLS approval callback). Two vectors
remained that could still land in the deadlocking input() fallback:

1. _spawn_background_review spawns a raw threading.Thread with no
   approval callback installed, so any dangerous-command guard the
   review agent trips falls back to input() -> deadlock against the
   parent's prompt_toolkit TUI (same class as delegate_task subagents,
   fixed in 023b1bff1 / #15491). Install a _bg_review_auto_deny
   callback at thread start, clear on finally.

2. prompt_dangerous_approval's fallback unconditionally spawned a
   daemon thread calling input() when approval_callback was None.
   That fallback can never succeed under prompt_toolkit because the
   user's Enter goes to pt's raw-mode stdin capture. Detect an active
   pt Application via get_app_or_none() and fail closed (deny + log)
   instead, so future threads that forget to install a callback
   degrade gracefully instead of hanging 60s invisibly.

Regression guards:
- tests/run_agent/test_background_review.py verifies the review
  worker thread sees a callable auto-deny callback mid-run and that
  the slot is cleared in the finally block.
- tests/tools/test_approval.py TestFailClosedUnderPromptToolkit
  verifies prompt_dangerous_approval returns 'deny' fast under a
  mocked pt Application, and that a real callback still wins over
  the guard.

run_agent.py

@@ -3247,6 +3247,21 @@ class AIAgent:
 
         def _run_review():
             import contextlib
+            # Install a non-interactive approval callback on this worker
+            # thread so any dangerous-command guard the review agent trips
+            # resolves to "deny" instead of falling back to input() -- which
+            # deadlocks against the parent's prompt_toolkit TUI (#15216).
+            # Same pattern as _subagent_auto_deny in tools/delegate_tool.py.
+            def _bg_review_auto_deny(command, description, **kwargs):
+                logger.warning(
+                    "Background review auto-denied dangerous command: %s (%s)",
+                    command, description,
+                )
+                return "deny"
+            try:
+                _set_approval_callback(_bg_review_auto_deny)
+            except Exception:
+                pass
             review_agent = None
             try:
                 with open(os.devnull, "w") as _devnull, \
@@ -3329,6 +3344,12 @@ class AIAgent:
                         review_agent.close()
                     except Exception:
                         pass
+                # Clear the approval callback on this bg-review thread so a
+                # recycled thread-id doesn't inherit a stale reference.
+                try:
+                    _set_approval_callback(None)
+                except Exception:
+                    pass
 
         t = threading.Thread(target=_run_review, daemon=True, name="bg-review")
         t.start()