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/core/test_external_urls.py

@@ -0,0 +1,32 @@
+"""
+Copyright 2024-2026 ChatterMate
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+"""
+
+from app.core.external_urls import external_base
+
+
+def test_default_when_unset(monkeypatch):
+    monkeypatch.delenv("TELEGRAM_API_BASE", raising=False)
+    assert external_base("TELEGRAM_API_BASE", "https://api.telegram.org") == "https://api.telegram.org"
+
+
+def test_override_without_trailing_slash(monkeypatch):
+    monkeypatch.setenv("TELEGRAM_API_BASE", "http://127.0.0.1:14020/telegram/")
+    assert external_base("TELEGRAM_API_BASE", "https://api.telegram.org") == "http://127.0.0.1:14020/telegram"
+
+
+def test_blank_override_falls_back(monkeypatch):
+    monkeypatch.setenv("TELEGRAM_API_BASE", "")
+    assert external_base("TELEGRAM_API_BASE", "https://api.telegram.org") == "https://api.telegram.org"