oidcauth: explicitly set test size to medium

The OIDC authorization tests can take 250s+ under race detection.
Make the `medium` size explicit (300s, 900s with race) to prevent
future regressions if the default changes.

Release note: None

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 2 people

pkg/ccl/oidcccl/BUILD.bazel

@@ -41,7 +41,7 @@ go_library(
 
 go_test(
     name = "oidcccl_test",
-    size = "small",
+    size = "medium",
     srcs = [
         "authentication_oidc_test.go",
         "authorization_oidc_test.go",