fix(swagger): avoid route collision, respect global Swagger flag, normalize app root

Address review feedback:
- Gate the APPLICATION_ROOT-aware Swagger/OpenAPI registration on
  FAB_API_SWAGGER_UI so the global Swagger disable still takes precedence.
- Suppress FAB's default OpenAPI/Swagger views (FAB_ADD_OPENAPI_VIEWS) when
  the prefix-aware variant is active, so the two don't register duplicate URL
  rules for /api/<version>/_openapi and /swagger/<version>.
- Normalize APPLICATION_ROOT (treat '/' as empty, strip trailing slash) so the
  Swagger UI spec URL is not built as a protocol-relative '//api/...' URL.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/unit_tests/openapi/test_manager.py

@@ -14,9 +14,25 @@
 # KIND, either express or implied.  See the License for the
 # specific language governing permissions and limitations
 # under the License.
+import pytest
 from marshmallow import Schema
 
-from superset.openapi.manager import resolver
+from superset.openapi.manager import normalize_app_root, resolver
+
+
+@pytest.mark.parametrize(
+    "app_root,expected",
+    [
+        ("/", ""),
+        ("", ""),
+        (None, ""),
+        ("/prefix", "/prefix"),
+        ("/prefix/", "/prefix"),
+        ("/nested/prefix/", "/nested/prefix"),
+    ],
+)
+def test_normalize_app_root(app_root: str | None, expected: str) -> None:
+    assert normalize_app_root(app_root) == expected
 
 
 def test_resolver_strips_schema_suffix() -> None: