test(tests): 增强SSL模式测试说明并调整类型标记

- 在SSL模式测试中添加详细注释，说明GaussDB支持的SSL模式及环境变量设置方法
- 为shapely类型测试添加gaussdb_skip标记，跳过GaussDB USTORE存储模式下不支持的PostGIS测试
- 改进测试代码的可读性和维护性

Author: 陈云亮

tests/test_sslmode.py

@@ -10,7 +10,15 @@
 
 @pytest.fixture(params=["require", "verify-ca"])
 def dsn(request):
-    """Retrieve DSN from environment variable based on SSL mode."""
+    """Retrieve DSN from environment variable based on SSL mode.
+
+    GaussDB SSL modes supported:
+    - require: Encrypted connection without certificate verification
+    - verify-ca: Encrypted connection with CA certificate verification
+
+    Set GAUSSDB_TEST_DSN with appropriate sslmode to run these tests:
+    export GAUSSDB_TEST_DSN="...sslmode=require..."
+    """
     dsn = os.environ.get("GAUSSDB_TEST_DSN")
     if not dsn:
         raise ValueError("GAUSSDB_TEST_DSN environment variable not set")

tests/types/test_shapely.py

@@ -25,6 +25,7 @@ def get_srid(obj):  # type: ignore[no-redef]
 pytestmark = [
     pytest.mark.postgis,
     pytest.mark.crdb("skip"),
+    pytest.mark.gaussdb_skip("PostGIS not supported on GaussDB USTORE storage mode"),
 ]
 
 SAMPLE_POINT = Point(1.2, 3.4)