test: add message_metadata unit tests

tests/agent/test_message_metadata.py

@@ -0,0 +1,23 @@
+from agent.message_metadata import append_message, stamp_message_timestamp
+
+
+def test_stamp_preserves_source_timestamp(monkeypatch):
+    monkeypatch.setattr("agent.message_metadata.wall_time", lambda: 999.0)
+    message = {"role": "user", "content": "hello", "timestamp": 123.0}
+
+    result = stamp_message_timestamp(message)
+
+    assert result is message
+    assert message["timestamp"] == 123.0
+
+
+def test_append_stamps_same_mapping_at_append_time(monkeypatch):
+    monkeypatch.setattr("agent.message_metadata.wall_time", lambda: 456.0)
+    messages = []
+    message = {"role": "tool", "content": "ok"}
+
+    result = append_message(messages, message)
+
+    assert result is message
+    assert messages == [message]
+    assert message["timestamp"] == 456.0