test(tui_gateway): stop reloading server module in fixture teardown (#34217)

tui_gateway.server registers two atexit hooks at module load time:
ThreadPoolExecutor shutdown (line 170) and _shutdown_sessions (line 336).
Three test files reloaded the module on each fixture teardown to reset
per-test state. Each reload re-runs module-level code, including the
atexit registrations — duplicates accumulate across the test session.

At pytest interpreter shutdown the duplicated atexit hooks race the
stderr buffer flush:

    Fatal Python error: _enter_buffered_busy: could not acquire lock
    for <_io.BufferedWriter name='<stderr>'> at interpreter shutdown,
    possibly due to daemon threads

pytest reports 'tests passed but the slice exited non-zero', and the
shard turns red on CI. Surfaced today on PR #34193's test slice 1
(204 files, 3572 tests passed, then Fatal Python error during exit).

Fix: drop importlib.reload(mod) from the three fixtures that have it.
Per-test reset is handled by clearing the mutable session dicts
(_sessions, _pending, _answers). _methods is also no longer cleared —
it's populated at module import time and would only be re-populated by
a reload, so clearing it without reload broke session.resume /
command.dispatch / slash.exec method registration across tests.

Affected fixtures:
- tests/tui_gateway/test_goal_command.py
- tests/tui_gateway/test_protocol.py
- tests/tui_gateway/test_review_summary_callback.py

The second reload in test_protocol.py at line 211 (reload of
tui_gateway.transport) is preserved — transport.py has no atexit hooks
or threads, so reload is safe there.

Tests: 84/84 in tests/tui_gateway/ pass cleanly with exit code 0; no
Fatal Python error at interpreter shutdown.

tests/tui_gateway/test_goal_command.py

@@ -44,11 +44,17 @@ def server(hermes_home):
     ):
         mod = importlib.import_module("tui_gateway.server")
         yield mod
+        # Reset module-level session state without re-importing. importlib.reload
+        # would re-register the module's atexit hooks (ThreadPoolExecutor
+        # shutdown, _shutdown_sessions); the duplicates race the stderr
+        # buffer at interpreter shutdown and surface as Fatal Python error:
+        # _enter_buffered_busy. Clearing the per-session dicts gives the
+        # next test a clean slate; _methods is NOT cleared because it's
+        # populated at module import time and re-registration only happens
+        # via reload (which we don't do).
         mod._sessions.clear()
         mod._pending.clear()
         mod._answers.clear()
-        mod._methods.clear()
-        importlib.reload(mod)
 
 
 @pytest.fixture()

tests/tui_gateway/test_protocol.py

@@ -30,11 +30,17 @@ def server():
         import importlib
         mod = importlib.import_module("tui_gateway.server")
         yield mod
+        # Reset module-level session state without re-importing. importlib.reload
+        # would re-register the module's atexit hooks (ThreadPoolExecutor
+        # shutdown, _shutdown_sessions); the duplicates race the stderr
+        # buffer at interpreter shutdown and surface as Fatal Python error:
+        # _enter_buffered_busy. Clearing the per-session dicts gives the
+        # next test a clean slate; _methods is NOT cleared because it's
+        # populated at module import time and re-registration only happens
+        # via reload (which we don't do).
         mod._sessions.clear()
         mod._pending.clear()
         mod._answers.clear()
-        mod._methods.clear()
-        importlib.reload(mod)
 
 
 @pytest.fixture()

tests/tui_gateway/test_review_summary_callback.py

@@ -34,11 +34,17 @@ def server():
 
         mod = importlib.import_module("tui_gateway.server")
         yield mod
+        # Reset module-level session state without re-importing. importlib.reload
+        # would re-register the module's atexit hooks (ThreadPoolExecutor
+        # shutdown, _shutdown_sessions); the duplicates race the stderr
+        # buffer at interpreter shutdown and surface as Fatal Python error:
+        # _enter_buffered_busy. Clearing the per-session dicts gives the
+        # next test a clean slate; _methods is NOT cleared because it's
+        # populated at module import time and re-registration only happens
+        # via reload (which we don't do).
         mod._sessions.clear()
         mod._pending.clear()
         mod._answers.clear()
-        mod._methods.clear()
-        importlib.reload(mod)
 
 
 def test_init_session_attaches_background_review_callback(server, monkeypatch):