fix(agent): bound per-session file state

tests/tools/test_file_state_store.py

@@ -0,0 +1,29 @@
+import pytest
+
+from nanobot.agent.tools.file_state import FileStateStore
+
+
+def test_file_state_store_evicts_least_recently_used_session() -> None:
+    store = FileStateStore(max_sessions=2)
+    first = store.for_session("first")
+    second = store.for_session("second")
+
+    assert store.for_session("first") is first
+    store.for_session("third")
+
+    assert store.for_session("first") is first
+    assert store.for_session("second") is not second
+
+
+def test_file_state_store_discards_reset_session() -> None:
+    store = FileStateStore()
+    previous = store.for_session("websocket:chat")
+
+    store.discard("websocket:chat")
+
+    assert store.for_session("websocket:chat") is not previous
+
+
+def test_file_state_store_requires_positive_capacity() -> None:
+    with pytest.raises(ValueError, match="max_sessions must be positive"):
+        FileStateStore(max_sessions=0)