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/utils/test_backoff.py

@@ -0,0 +1,35 @@
+"""
+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.utils.backoff import next_attempt_delay
+
+POLICY = {"base_seconds": 60, "max_seconds": 3600, "jitter_seconds": 0}
+
+
+def test_exponential_then_capped():
+    assert next_attempt_delay(1, **POLICY) == 120
+    assert next_attempt_delay(3, **POLICY) == 480
+    assert next_attempt_delay(10, **POLICY) == 3600
+
+
+def test_a_longer_retry_after_wins():
+    assert next_attempt_delay(1, retry_after_seconds=900, **POLICY) == 900
+    assert next_attempt_delay(3, retry_after_seconds=5, **POLICY) == 480
+
+
+def test_jitter_only_adds():
+    delay = next_attempt_delay(1, base_seconds=60, max_seconds=3600, jitter_seconds=30)
+    assert 120 <= delay <= 150