fix: 将52号接口范围限定为GPU worker

撤回52号网关路由、图片字段改名和mock改动，gateway目录恢复a3fddbb。worker保持base64和null响应，更新接口说明和回归测试。

验证：worker API、IP限制与网关兼容性共54项测试通过。

tests/test_gateway_nullable_images.py

@@ -1,4 +1,4 @@
-"""Nullable/deprecated worker images retain their documented gateway keys."""
+"""Keep the gateway's historical image-field conversion unchanged."""
 import asyncio
 import json
 from types import SimpleNamespace
@@ -11,14 +11,12 @@ from starlette.requests import Request
 from gateway.forward import rewrite_base64_to_url
 
 
-def test_nullable_image_fields_keep_external_names(tmp_path):
+def test_nullable_image_fields_keep_original_names(tmp_path):
     value = {"hairline_images": [{"image_high_base64": None, "image_low_base64": None,
                                    "grown_image_base64": None}],
              "high_hairline_center_point": None, "face_measure": None}
-    actual = rewrite_base64_to_url(value, "https://example.test", str(tmp_path), rewrite_null_images=True)
-    assert actual == {"hairline_images": [{"image_high_url": None, "image_low_url": None,
-                                           "grown_image_url": None}],
-                      "high_hairline_center_point": None, "face_measure": None}
+    actual = rewrite_base64_to_url(value, "https://example.test", str(tmp_path))
+    assert actual == value
     assert not list(tmp_path.iterdir())
 
 
@@ -30,23 +28,18 @@ def test_nonempty_images_still_rewrite_alongside_null_fields(tmp_path):
     Image.new("RGBA", (2, 2), (255, 255, 255, 0)).save(buf, format="PNG")
     value = {"image_middle_base64": base64.b64encode(buf.getvalue()).decode(),
              "image_high_base64": None}
-    actual = rewrite_base64_to_url(value, "https://example.test", str(tmp_path), rewrite_null_images=True)
+    actual = rewrite_base64_to_url(value, "https://example.test", str(tmp_path))
     assert actual["image_middle_url"].startswith("https://example.test/static/annotations/")
-    assert actual["image_high_url"] is None
+    assert actual["image_high_base64"] is None
+    assert "image_high_url" not in actual
     assert len(list(tmp_path.iterdir())) == 1
 
 
-def test_legacy_nullable_image_field_names_remain_unchanged(tmp_path):
-    value = {"hairline_images": [{"grown_image_base64": None}], "face_measure": None}
-    assert rewrite_base64_to_url(value, "https://example.test", str(tmp_path)) == value
-    assert not list(tmp_path.iterdir())
-
-
 @pytest.mark.parametrize("path,expected_key", [
     ("/api/v1/hairline/generate", "grown_image_base64"),
-    ("/api/v1/hairline/generate-52", "grown_image_url"),
+    ("/api/v1/hair/grow", "grown_image_base64"),
 ])
-def test_proxy_rewrites_null_images_only_for_interface52(tmp_path, monkeypatch, path, expected_key):
+def test_proxy_keeps_original_null_image_fields(tmp_path, monkeypatch, path, expected_key):
     import gateway.config as config
     import gateway.forward as forward