add explain module

sentence_api/tests/test_ai_tutor.py

@@ -0,0 +1,57 @@
+import pytest
+
+from sentence_api.ai_tutor import AITutor, AITutorError
+
+
+class FakeResponse:
+    def __init__(self, status_code=200, body=None):
+        self.status_code = status_code
+        self._body = body or {}
+
+    def json(self):
+        if self._body is None:
+            raise ValueError("invalid JSON")
+        return self._body
+
+
+def test_ai_tutor_parses_chat_completion(monkeypatch):
+    requests_payloads = []
+
+    def fake_post(endpoint, headers, json, timeout):
+        requests_payloads.append((endpoint, headers, json, timeout))
+        return FakeResponse(
+            body={
+                "choices": [
+                    {"message": {"content": " 这句话的意思是…… "}}
+                ]
+            }
+        )
+
+    monkeypatch.setattr("sentence_api.ai_tutor.requests.post", fake_post)
+    tutor = AITutor("https://ai.test/chat", "secret", "test-model", 1.5)
+
+    explanation = tutor.explain(
+        "The early bird catches the worm.",
+        course_title="English idioms",
+        sentence_index=1,
+        language="en",
+    )
+
+    endpoint, headers, payload, timeout = requests_payloads[0]
+    assert explanation == "这句话的意思是……"
+    assert endpoint == "https://ai.test/chat"
+    assert headers["Authorization"] == "Bearer secret"
+    assert payload["model"] == "test-model"
+    assert "The early bird catches the worm." in payload["messages"][1]["content"]
+    assert timeout == 1.5
+
+
+def test_ai_tutor_rejects_unexpected_response(monkeypatch):
+    monkeypatch.setattr(
+        "sentence_api.ai_tutor.requests.post",
+        lambda *args, **kwargs: FakeResponse(body={"choices": []}),
+    )
+    tutor = AITutor("https://ai.test/chat", "secret", "test-model", 1)
+
+    with pytest.raises(AITutorError):
+        tutor.explain("Hello.")