fix(session): cap messages at persistence boundary

Bind SessionManager saves to the existing raw archive path so SDK imports and other bypass saves cannot persist more than the file cap without archiving unconsolidated overflow.

Add an SDK regression test that exercises the real ingest path.

Refs #4787

tests/test_nanobot_facade.py

@@ -35,6 +35,7 @@ from nanobot.runtime_context import (
     RuntimeContextBlock,
     append_runtime_context,
 )
+from nanobot.session.manager import FILE_MAX_MESSAGES
 from nanobot.utils.llm_runtime import runtime_from_provider_snapshot
 
 
@@ -1205,6 +1206,27 @@ async def test_sessions_ingest_imports_transcript_without_running_model(tmp_path
     assert reloaded.messages == snapshot.messages
 
 
+@pytest.mark.asyncio
+async def test_sessions_ingest_archives_overflow_at_persistence_boundary(tmp_path):
+    config_path = _write_config(tmp_path)
+    bot = Nanobot.from_config(config_path, workspace=tmp_path)
+
+    snapshot = await bot.sessions.ingest(
+        "sdk:overflow",
+        [
+            {"role": "user", "content": f"message-{index}"}
+            for index in range(FILE_MAX_MESSAGES + 1)
+        ],
+    )
+
+    assert len(snapshot.messages) == FILE_MAX_MESSAGES
+    assert snapshot.messages[0]["content"] == "message-1"
+    history = bot.memory.read_history(session_key="sdk:overflow")
+    assert len(history) == 1
+    assert "[RAW] 1 messages" in history[0]["content"]
+    assert "message-0" in history[0]["content"]
+
+
 @pytest.mark.asyncio
 async def test_sessions_ingest_validates_message_shape(tmp_path):
     config_path = _write_config(tmp_path)