fix: keep ACP stdout protocol-clean

Route AIAgent print output to stderr via _print_fn for ACP stdio sessions.
Gate quiet-mode spinner startup on _should_start_quiet_spinner() so JSON-RPC
on stdout isn't corrupted. Child agents inherit the redirect.

Co-authored-by: Git-on-my-level <Git-on-my-level@users.noreply.github.com>

tests/test_run_agent.py

@@ -2612,6 +2612,24 @@ def test_aiagent_uses_copilot_acp_client():
     assert mock_acp_client.call_args.kwargs["args"] == ["--acp", "--stdio"]
 
 
+def test_quiet_spinner_allowed_with_explicit_print_fn(agent):
+    agent._print_fn = lambda *_a, **_kw: None
+    with patch.object(run_agent.sys.stdout, "isatty", return_value=False):
+        assert agent._should_start_quiet_spinner() is True
+
+
+def test_quiet_spinner_allowed_on_real_tty(agent):
+    agent._print_fn = None
+    with patch.object(run_agent.sys.stdout, "isatty", return_value=True):
+        assert agent._should_start_quiet_spinner() is True
+
+
+def test_quiet_spinner_suppressed_on_non_tty_without_print_fn(agent):
+    agent._print_fn = None
+    with patch.object(run_agent.sys.stdout, "isatty", return_value=False):
+        assert agent._should_start_quiet_spinner() is False
+
+
 def test_is_openai_client_closed_honors_custom_client_flag():
     assert AIAgent._is_openai_client_closed(SimpleNamespace(is_closed=True)) is True
     assert AIAgent._is_openai_client_closed(SimpleNamespace(is_closed=False)) is False