test: standardize async tests on pytest-asyncio auto mode (#338)

* chore: add pytest-asyncio in auto mode

* test: migrate channel and stream tests to native async

Convert run_async() wrapper tests to plain 'async def test_*' under
pytest-asyncio auto mode. collect_events() in stream_v3_fakes becomes a
coroutine awaited at every call site.

* test: migrate command and model/middleware tests to native async

Convert run_async() wrappers (import, alias, and fixture forms) to plain
'async def test_*'. Multi-call tests merge onto one loop as sequential
awaits; none asserted on loop identity.

* test: migrate TUI, notifier, gateway, and session tests to native async

TUI/notifier/gateway files convert run_async wrappers to plain async
tests. test_sessions.py's unittest.TestCase classes move to
unittest.IsolatedAsyncioTestCase (pytest-asyncio does not await async
methods on plain TestCase; converting blindly would have made ~70 tests
silently vacuous). Its setUpClass keeps a one-shot asyncio.run() since
IsolatedAsyncioTestCase has no async class-level hook. TestLoadingWidget
in test_tui_widgets.py drops its TestCase base for the same reason.

* test: replace direct asyncio.run() calls with native async tests

Convert tests that called asyncio.run() (directly or via a local _run
helper) to plain 'async def test_*'; delete the local helpers.

* test: drop undeclared anyio markers and delete run_async helper

The @pytest.mark.anyio tests relied on anyio being a transitive dep of
httpx; auto-mode pytest-asyncio collects them natively. run_async() and
its fixture are unreferenced after the migration, so remove them —
pytest-asyncio's per-test loop teardown covers the pending-task
cancellation the helper existed for (verified: full suite runs with no
'Event loop is closed' errors or destroyed-task warnings).

tests/test_exit_command.py

@@ -2,11 +2,9 @@
 
 from unittest.mock import MagicMock
 
-from tests.conftest import run_async as _run
-
 
 class TestExitCommand:
-    def test_execute_calls_force_quit(self):
+    async def test_execute_calls_force_quit(self):
         from EvoScientist.commands.base import CommandContext
         from EvoScientist.commands.implementation.session import ExitCommand
 
@@ -17,7 +15,7 @@ class TestExitCommand:
             ui=ui,
         )
         cmd = ExitCommand()
-        _run(cmd.execute(ctx, []))
+        await cmd.execute(ctx, [])
         ui.force_quit.assert_called_once()
 
     def test_aliases_registered(self):