fix(agent): skip empty multimodal user entries after runtime-context strip

tests/test_loop_save_turn.py

@@ -0,0 +1,41 @@
+from nanobot.agent.context import ContextBuilder
+from nanobot.agent.loop import AgentLoop
+from nanobot.session.manager import Session
+
+
+def _mk_loop() -> AgentLoop:
+    loop = AgentLoop.__new__(AgentLoop)
+    loop._TOOL_RESULT_MAX_CHARS = 500
+    return loop
+
+
+def test_save_turn_skips_multimodal_user_when_only_runtime_context() -> None:
+    loop = _mk_loop()
+    session = Session(key="test:runtime-only")
+    runtime = ContextBuilder._RUNTIME_CONTEXT_TAG + "\nCurrent Time: now (UTC)"
+
+    loop._save_turn(
+        session,
+        [{"role": "user", "content": [{"type": "text", "text": runtime}]}],
+        skip=0,
+    )
+    assert session.messages == []
+
+
+def test_save_turn_keeps_image_placeholder_after_runtime_strip() -> None:
+    loop = _mk_loop()
+    session = Session(key="test:image")
+    runtime = ContextBuilder._RUNTIME_CONTEXT_TAG + "\nCurrent Time: now (UTC)"
+
+    loop._save_turn(
+        session,
+        [{
+            "role": "user",
+            "content": [
+                {"type": "text", "text": runtime},
+                {"type": "image_url", "image_url": {"url": "data:image/png;base64,abc"}},
+            ],
+        }],
+        skip=0,
+    )
+    assert session.messages[0]["content"] == [{"type": "text", "text": "[image]"}]