feat: add WeChat unlimited mini program code generator

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/tests/test_wxacode.py

@@ -0,0 +1,47 @@
+from unittest.mock import MagicMock, patch
+
+from app.services.wechat_mini_auth import get_unlimited_wxacode, WechatMiniAuthError
+
+
+def test_get_unlimited_wxacode_returns_png_bytes():
+    token_resp = MagicMock()
+    token_resp.raise_for_status = MagicMock()
+    token_resp.json.return_value = {"access_token": "tok"}
+
+    code_resp = MagicMock()
+    code_resp.raise_for_status = MagicMock()
+    code_resp.headers = {"Content-Type": "image/jpeg"}
+    code_resp.content = b"\xff\xd8fake"
+
+    with patch("app.services.wechat_mini_auth._require_mini_credentials", return_value=("app", "sec")):
+        with patch("app.services.wechat_mini_auth.httpx.Client") as Client:
+            client = MagicMock()
+            Client.return_value.__enter__.return_value = client
+            client.get.return_value = token_resp
+            client.post.return_value = code_resp
+            raw = get_unlimited_wxacode(scene="abc", page="pages/login/web-qr/index")
+    assert raw.startswith(b"\xff\xd8")
+
+
+def test_get_unlimited_wxacode_json_error():
+    token_resp = MagicMock()
+    token_resp.raise_for_status = MagicMock()
+    token_resp.json.return_value = {"access_token": "tok"}
+
+    code_resp = MagicMock()
+    code_resp.raise_for_status = MagicMock()
+    code_resp.headers = {"Content-Type": "application/json"}
+    code_resp.content = b'{"errcode":40001,"errmsg":"fail"}'
+    code_resp.json.return_value = {"errcode": 40001, "errmsg": "fail"}
+
+    with patch("app.services.wechat_mini_auth._require_mini_credentials", return_value=("app", "sec")):
+        with patch("app.services.wechat_mini_auth.httpx.Client") as Client:
+            client = MagicMock()
+            Client.return_value.__enter__.return_value = client
+            client.get.return_value = token_resp
+            client.post.return_value = code_resp
+            try:
+                get_unlimited_wxacode(scene="abc", page="pages/login/web-qr/index")
+                assert False, "expected error"
+            except WechatMiniAuthError:
+                pass