test: isolate the prompt_toolkit app session so UI tests run on Windows

tests/conftest.py

@@ -1,10 +1,32 @@
 from __future__ import annotations
 
 import pytest
+from prompt_toolkit.application import create_app_session
+from prompt_toolkit.input import create_pipe_input
+from prompt_toolkit.output import DummyOutput
 
 from fast_agent.integrations import herdr_lifecycle
 
 
+@pytest.fixture(autouse=True)
+def isolate_prompt_toolkit_app_session():
+    """Give every test its own prompt_toolkit app session, detached from the terminal.
+
+    Building a prompt_toolkit Application binds the ambient AppSession's output,
+    and constructing that output probes the terminal. Under pytest's captured
+    stdout on Windows the probe raises NoConsoleScreenBufferError, so every test
+    that builds UI fails there — while Linux CI never sees it, because the POSIX
+    Vt100 output has no equivalent probe.
+
+    Binding a DummyOutput and a pipe input makes the session explicit rather than
+    ambient, which is also what keeps these tests from touching the real terminal
+    when they are run locally.
+    """
+    with create_pipe_input() as pipe_input:
+        with create_app_session(input=pipe_input, output=DummyOutput()):
+            yield
+
+
 @pytest.fixture(autouse=True)
 def isolate_herdr_lifecycle(monkeypatch: pytest.MonkeyPatch):
     """Prevent tests run inside Herdr from reporting against the developer's pane."""