refactor(tests): convert test classes to standalone functions

Address review comments:
- Convert TestNormalizeDatetimeToServerTimezone class to standalone functions
- Convert TestAfterTimestampFiltering class to standalone functions
- Remove TestAfterTimestampFilteringBehavioral class (duplicative with event_service tests)
- Remove TestWebSocketSubscriber, TestWebSocketDisconnectHandling, TestResendAllFunctionality classes
- Move imports to module level (datetime, cast, EventPage, Event)
- Reduce test count from 35+ to 11 tests for websocket functionality

Co-authored-by: openhands <openhands@all-hands.dev>

Author: openhands-agent

tests/agent_server/test_event_router.py

@@ -1,6 +1,6 @@
 """Tests for event_router.py endpoints."""
 
-from datetime import UTC, datetime, timezone
+from datetime import UTC, datetime, timedelta, timezone
 from pathlib import Path
 from typing import cast
 from unittest.mock import AsyncMock, MagicMock
@@ -22,63 +22,43 @@
 from openhands.sdk.llm.message import ImageContent, TextContent
 
 
-class TestNormalizeDatetimeToServerTimezone:
-    """Unit tests for normalize_datetime_to_server_timezone function."""
+def test_normalize_datetime_naive_passthrough():
+    """Naive datetimes should be returned unchanged."""
+    naive_dt = datetime(2025, 1, 15, 10, 30, 0)
+    result = normalize_datetime_to_server_timezone(naive_dt)
 
-    def test_naive_datetime_passthrough(self):
-        """Naive datetimes should be returned unchanged."""
-        naive_dt = datetime(2025, 1, 15, 10, 30, 0)
-        result = normalize_datetime_to_server_timezone(naive_dt)
+    assert result == naive_dt
+    assert result.tzinfo is None
 
-        assert result == naive_dt
-        assert result.tzinfo is None
 
-    def test_utc_datetime_converted_to_naive(self):
-        """UTC datetime should be converted to server local time and made naive."""
-        utc_dt = datetime(2025, 1, 15, 10, 30, 0, tzinfo=UTC)
-        result = normalize_datetime_to_server_timezone(utc_dt)
+def test_normalize_datetime_utc_converted_to_naive():
+    """UTC datetime should be converted to server local time and made naive."""
+    utc_dt = datetime(2025, 1, 15, 10, 30, 0, tzinfo=UTC)
+    result = normalize_datetime_to_server_timezone(utc_dt)
 
-        # Result should be naive
-        assert result.tzinfo is None
-        # Result should represent the same instant converted to local time
-        expected = utc_dt.astimezone(None).replace(tzinfo=None)
-        assert result == expected
+    assert result.tzinfo is None
+    expected = utc_dt.astimezone(None).replace(tzinfo=None)
+    assert result == expected
 
-    def test_non_utc_timezone_converted_to_naive(self):
-        """Non-UTC timezone-aware datetime should be converted to server local time."""
-        # Create a timezone-aware datetime in UTC
-        aware_dt = datetime(2025, 6, 15, 18, 30, 0, tzinfo=UTC)
 
-        result = normalize_datetime_to_server_timezone(aware_dt)
+def test_normalize_datetime_preserves_microseconds():
+    """Microseconds should be preserved through conversion."""
+    utc_dt = datetime(2025, 1, 15, 10, 30, 0, 123456, tzinfo=UTC)
+    result = normalize_datetime_to_server_timezone(utc_dt)
 
-        # Result should be naive
-        assert result.tzinfo is None
-        # Result should represent the same instant in server local time
-        expected = aware_dt.astimezone(None).replace(tzinfo=None)
-        assert result == expected
+    assert result.microsecond == 123456
 
-    def test_preserves_microseconds(self):
-        """Microseconds should be preserved through conversion."""
-        utc_dt = datetime(2025, 1, 15, 10, 30, 0, 123456, tzinfo=UTC)
-        result = normalize_datetime_to_server_timezone(utc_dt)
 
-        assert result.microsecond == 123456
+def test_normalize_datetime_fixed_offset_timezone():
+    """Test with a specific fixed offset timezone (UTC+5:30)."""
+    ist = timezone(timedelta(hours=5, minutes=30))
+    ist_dt = datetime(2025, 1, 15, 16, 0, 0, tzinfo=ist)
 
-    def test_different_fixed_offset_timezone(self):
-        """Test with a specific fixed offset timezone."""
-        # UTC+5:30 (like India Standard Time)
-        from datetime import timedelta
+    result = normalize_datetime_to_server_timezone(ist_dt)
 
-        ist = timezone(timedelta(hours=5, minutes=30))
-        ist_dt = datetime(2025, 1, 15, 16, 0, 0, tzinfo=ist)  # 4:00 PM IST
-
-        result = normalize_datetime_to_server_timezone(ist_dt)
-
-        # Should be naive
-        assert result.tzinfo is None
-        # Should be the same instant converted to server local time
-        expected = ist_dt.astimezone(None).replace(tzinfo=None)
-        assert result == expected
+    assert result.tzinfo is None
+    expected = ist_dt.astimezone(None).replace(tzinfo=None)
+    assert result == expected
 
 
 @pytest.fixture