tests: mark trailing slash test as xfail

Pydantic's AnyUrl incorrectly adds trailing slashes to base URLs without
paths. This behavior is being fixed in pydantic-core PR #1719 and will
be available in Pydantic 2.12+.

Instead of working around the issue, mark the test as expected to fail.
This documents the known bug and will automatically alert when Pydantic
fixes it (the test will XPASS with xfail_strict=true in pyproject.toml).

Ref: pydantic/pydantic-core#1719

Author: felixweinberger

tests/client/test_auth.py

@@ -836,16 +836,20 @@ async def test_token_exchange_error_basic(self, oauth_provider, oauth_client_inf
         "revocation_endpoint",
     ),
     (
-        # TODO(Marcelo): Since we are using `AnyUrl`, the trailing slash is always added.
-        # pytest.param(
-        #     "https://auth.example.com",
-        #     "https://auth.example.com/docs",
-        #     "https://auth.example.com/authorize",
-        #     "https://auth.example.com/token",
-        #     "https://auth.example.com/register",
-        #     "https://auth.example.com/revoke",
-        #     id="simple-url",
-        # ),
+        # Pydantic's AnyUrl incorrectly adds trailing slash to base URLs
+        # This is being fixed in https://github.com/pydantic/pydantic-core/pull/1719 (Pydantic 2.12+)
+        pytest.param(
+            "https://auth.example.com",
+            "https://auth.example.com/docs",
+            "https://auth.example.com/authorize",
+            "https://auth.example.com/token",
+            "https://auth.example.com/register",
+            "https://auth.example.com/revoke",
+            id="simple-url",
+            marks=pytest.mark.xfail(
+                reason="Pydantic AnyUrl adds trailing slash to base URLs - fixed in Pydantic 2.12+"
+            ),
+        ),
         pytest.param(
             "https://auth.example.com/",
             "https://auth.example.com/docs",