Fix TiDB unit test failures

- Fix hostname mocking issues by setting reported_hostname directly on instances
- Fix JSON serialization error by using proper config class instead of MagicMock
- Ensure cloud_metadata is a dict not MagicMock to prevent serialization issues

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: takaidohigasi

mysql/tests/test_query_activity.py

@@ -871,16 +871,21 @@ def test_tidb_activity_multiple_nodes():
     check._events_wait_current_enabled = False  # Force TiDB path
     # Mock the property getter
     check.events_wait_current_enabled = False
+    check.reported_hostname = 'test-host'  # Set this to avoid MagicMock in JSON
     check.database_monitoring_query_activity = mock.MagicMock()
     check.histogram = mock.MagicMock()
     check._get_debug_tags = mock.MagicMock(return_value=[])
 
-    config = mock.MagicMock()
-    config.activity_config = {'enabled': True, 'collection_interval': 10}
-    config.obfuscator_options = {'replace_digits': True}
-    config.cloud_metadata = {}
-    config.service = 'test-tidb'
-    config.min_collection_interval = 1
+    # Create a proper config object instead of MagicMock to avoid JSON serialization issues
+    class MockConfig:
+        def __init__(self):
+            self.activity_config = {'enabled': True, 'collection_interval': 10}
+            self.obfuscator_options = {'replace_digits': True}
+            self.cloud_metadata = {}
+            self.service = 'test-tidb'
+            self.min_collection_interval = 1
+
+    config = MockConfig()
 
     activity_collector = MySQLActivity(check, config, {})
     # Initialize _tags which is normally set by the parent class