chore(librarian): add exception for google.cloud.orgpolicy (#14779)

This PR fixes the following stack trace which caused the build to fail
for `google-cloud-org-policy` in `librarian generate` PR
#14776

```
Traceback (most recent call last):
  File "/app/./cli.py", line 1043, in handle_build
    _verify_library_namespace(library_id, repo)
  File "/app/./cli.py", line 992, in _verify_library_namespace
    raise ValueError(
ValueError: The namespace `google.cloud.orgpolicy` for `google-cloud-org-policy` must be one of ['google', 'google.ads', 'google.ai', 'google.analytics', 'google.apps', 'google.cloud', 'google.geo', 'google.maps', 'google.shopping', 'grafeas', 'google.area120', 'google.api', 'google.apps.script', 'google.apps.script.type', 'google.cloud.alloydb', 'google.cloud.billing', 'google.cloud.devtools', 'google.cloud.gkeconnect', 'google.cloud.gkehub_v1', 'google.cloud.orchestration.airflow', 'google.cloud.security', 'google.cloud.video', 'google.cloud.workflows', 'google.iam', 'google.gapic', 'google.identity.accesscontextmanager', 'google.logging', 'google.monitoring', 'google.rpc'].

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/app/./cli.py", line 1482, in <module>
    args.func(librarian=args.librarian, repo=args.repo)
  File "/app/./cli.py", line 1047, in handle_build
    raise ValueError("Build failed.") from e
ValueError: Build failed.
```

Author: parthea

.generator/cli.py

@@ -937,6 +937,7 @@ def _verify_library_namespace(library_id: str, repo: str):
         "google.cloud.gkeconnect",
         "google.cloud.gkehub_v1",
         "google.cloud.orchestration.airflow",
+        "google.cloud.orgpolicy",
         "google.cloud.security",
         "google.cloud.video",
         "google.cloud.workflows",