Fix resize and poller races; add coverage

- Fix resize message handling when session already exists
- Guard poller selector.modify against removed fds
- Handle send_bytes race when master_fd closes
- Add tests for resize edge case, poller write KeyError, send_bytes race

tests/test_local_server_unit.py

@@ -679,6 +679,19 @@ class TestLocalServerMoreCoverage:
         session.send_bytes.assert_awaited_once_with(b"")
 
     @pytest.mark.asyncio
+    @pytest.mark.asyncio
+    async def test_dispatch_ws_message_resize_existing_session_flag_false(self, server_with_no_apps, monkeypatch):
+        session = MagicMock()
+        session.set_terminal_size = AsyncMock()
+        monkeypatch.setattr(server_with_no_apps.session_manager, "get_session_by_route_key", lambda _rk: session)
+
+        ws = MagicMock()
+        created = await server_with_no_apps._dispatch_ws_message(
+            ["resize", {"width": 100, "height": 50}], "rk", ws, False
+        )
+        assert created is False
+        session.set_terminal_size.assert_awaited_once_with(100, 50)
+
     async def test_dispatch_ws_message_resize_updates_existing_session(self, server_with_no_apps, monkeypatch):
         session = MagicMock()
         session.set_terminal_size = AsyncMock()

tests/test_poller.py

@@ -71,6 +71,17 @@ class TestPoller:
             # Should not raise
             poller.remove_file(999)
 
+    @pytest.mark.asyncio
+    async def test_write_handles_removed_fd(self):
+        poller = Poller()
+        poller._loop = asyncio.get_event_loop()
+
+        with patch.object(poller._selector, "register"):
+            poller.add_file(42)
+
+        with patch.object(poller._selector, "modify", side_effect=KeyError()):
+            await poller.write(42, b"test")
+
     @pytest.mark.asyncio
     async def test_write_creates_queue(self):
         """Test that write creates a write queue if needed."""

tests/test_terminal_session.py

@@ -288,6 +288,18 @@ class TestTerminalSession:
 
         mock_exit.assert_called_once_with(1)
 
+    @pytest.mark.asyncio
+    async def test_send_bytes_handles_closed_fd(self):
+        from textual_webterm.terminal_session import TerminalSession
+
+        poller = MagicMock()
+        poller.write = AsyncMock(side_effect=KeyError)
+        session = TerminalSession(poller, "sid", "bash")
+        session.master_fd = 10
+
+        ok = await session.send_bytes(b"test")
+        assert ok is False
+
     @pytest.mark.asyncio
     async def test_run_reads_from_poller_and_closes(self):
         from textual_webterm.terminal_session import TerminalSession