test: 补充知识处理、检索与 Agent 回归用例

覆盖切块、向量检索、工具调用和历史预算；模型相关用例使用桩。

backend/tests/README.md

@@ -0,0 +1,39 @@
+# Backend tests
+
+```bash
+cd backend && ./venv/bin/python -m pytest tests/ -v
+```
+
+Use the **project venv** (`./venv/bin/python`), not a system or conda Python —
+dependencies from `requirements.txt` such as `paramiko` are only installed
+there, and a wrong interpreter shows up as unrelated `ModuleNotFoundError`
+failures.
+
+## Matching CI locally: `NO_ENTERPRISE=1`
+
+CI checks out with `actions/checkout@v3` and **no `submodules: recursive`**, so
+`app/enterprise` is empty there and the app runs in community mode. A local
+checkout usually has the submodule populated, which switches on plan and
+subscription gating — and around 30 API tests then return a `403` they never
+see in CI.
+
+Those failures are environmental, not real. To get a result comparable to CI:
+
+```bash
+cd backend && NO_ENTERPRISE=1 ./venv/bin/python -m pytest tests/ -v
+```
+
+This installs an import blocker (see the top of `conftest.py`) that makes
+`app.enterprise` raise `ModuleNotFoundError`, exactly as an un-checked-out
+submodule does, so the `try: import app.enterprise` guards and
+`pytest.importorskip` fall back to community mode the same way.
+
+Run **without** the flag when you are working on enterprise code and want the
+gating active.
+
+## Live agent tests
+
+`tests_live/` runs the chat agent against the real model and local Postgres to
+catch prompt-effectiveness regressions the unit suite cannot see. It sits
+outside `testpaths`, so it is never collected here or in CI. See
+[tests_live/README.md](../tests_live/README.md).