Fix truncate_messages persisting an inflated message_count (#2052)

truncate_messages deletes db_messages[keep_count:] (a no-op when
keep_count >= the real message total) then unconditionally wrote
db_session.message_count = keep_count. When keep_count exceeds the
number of messages that actually exist — e.g. the manage_session AI
tool defaults keep_count to 10, and the HTTP truncate endpoint passes
any client value — the persisted count is set too high (10 on a
3-message session), diverging from the real row count. That column
gates lazy DB-hydration in get_session (message_count > 0) and is
surfaced to the history UI, so it is correctness-relevant. Clamp to
min(keep_count, len(db_messages)); the in-memory slice already caps
naturally.

tests/test_truncate_message_count_regression.py

@@ -0,0 +1,59 @@
+"""Regression: truncate_messages must not set message_count above the real
+number of messages when keep_count exceeds the message total.
+
+The AI tool layer (src/ai_interaction.py manage_session action='truncate')
+defaults keep_count=10, so a short session (say 3 messages) gets truncated
+with keep_count=10. The DB has only 3 rows left, but truncate_messages used to
+write db_session.message_count = keep_count (=10), leaving the persisted count
+inconsistent with the actual rows. get_session relies on message_count>0 to
+decide whether to lazily hydrate from the DB, so an inflated count is a latent
+correctness hazard.
+"""
+import os
+import tempfile
+
+
+def _make_manager():
+    db_fd, db_path = tempfile.mkstemp(suffix=".db")
+    os.close(db_fd)
+    os.environ["DATABASE_URL"] = f"sqlite:///{db_path}"
+
+    # Import after DATABASE_URL is set so the engine binds to the temp DB.
+    import importlib
+    import core.database as database
+    importlib.reload(database)
+    database.Base.metadata.create_all(bind=database.engine)
+
+    import core.session_manager as sm_mod
+    importlib.reload(sm_mod)
+    return sm_mod.SessionManager(), database, sm_mod
+
+
+def test_truncate_keep_count_exceeds_total_does_not_inflate_count():
+    from core.models import ChatMessage
+
+    sm, database, sm_mod = _make_manager()
+    sid = "short-session"
+    sm.create_session(session_id=sid, name="t", endpoint_url="x",
+                      model="m", rag=False, owner="u")
+    for i in range(3):
+        sm.add_message(sid, ChatMessage("user", f"msg{i}"))
+
+    # AI default keep_count is 10 — larger than the 3 real messages.
+    assert sm.truncate_messages(sid, 10) is True
+
+    db = database.SessionLocal()
+    try:
+        DbSession = database.Session
+        DbChatMessage = database.ChatMessage
+        rows = db.query(DbChatMessage).filter(
+            DbChatMessage.session_id == sid).count()
+        db_session = db.query(DbSession).filter(DbSession.id == sid).first()
+        # Nothing should have been deleted (only 3 messages exist).
+        assert rows == 3
+        # message_count must reflect the real number of rows, not keep_count.
+        assert db_session.message_count == 3, (
+            f"message_count={db_session.message_count} but only {rows} rows exist"
+        )
+    finally:
+        db.close()