Roles metadata is missing

Hi Spanner team, I hit this with the emulator while testing database roles.

## Summary

The emulator accepts `CREATE ROLE`, but the role does not show up in role metadata. Real Spanner shows the same role in `GetDatabaseDdl`, `INFORMATION_SCHEMA.ROLES`, and `databaseRoles.list`.

## Environment

- Emulator version: 1.5.53

## Repro

```sh
SPANNER_EMULATOR_HOST=localhost:9010 \
gcloud spanner databases create db \
--instance=test-instance \
--project=my-project

SPANNER_EMULATOR_HOST=localhost:9010 \
gcloud spanner databases ddl update db \
--instance=test-instance \
--project=my-project \
--ddl='CREATE ROLE my-super-role'

SPANNER_EMULATOR_HOST=localhost:9010 \
gcloud spanner databases ddl update db \
--instance=test-instance \
--project=my-project \
--ddl='CREATE SCHEMA s'

Schema updating...
.....done.
```

## Actual

`GetDatabaseDdl` shows the schema but not the role:

```sh
SPANNER_EMULATOR_HOST=localhost:9010 \
gcloud spanner databases ddl describe db \
--instance=test-instance \
--project=my-project
```

```sql
CREATE SCHEMA s;
```

`databaseRoles.list` returns 501:

```sh
SPANNER_EMULATOR_HOST=localhost:9010 \
gcloud spanner databases roles list \
--instance=test-instance \
--database=db \
--project=my-project \
--format=json
```

```text
status: 501
content: {"code":12}
```

`INFORMATION_SCHEMA.ROLES` returns 500:

```sh
SPANNER_EMULATOR_HOST=localhost:9010 \
gcloud spanner databases execute-sql db \
--instance=test-instance \
--project=my-project \
--sql='SELECT ROLE_NAME FROM INFORMATION_SCHEMA.ROLES ORDER BY ROLE_NAME' \
--format=json
```

```text
status: 500
content: {"code": 13, "message": "failed to marshal error message"}
```

```sh
SPANNER_EMULATOR_HOST=localhost:9010 \
gcloud spanner databases execute-sql db \
--instance=test-instance \
--project=my-project \
--sql='SELECT SCHEMA_NAME FROM INFORMATION_SCHEMA.SCHEMATA' \
--format=json
```

```json
{
"rows": [
[""],
["INFORMATION_SCHEMA"],
["SPANNER_SYS"],
["s"]
]
}
```

## Expected

After `CREATE ROLE my-super-role` succeeds, `my-super-role` should be visible through the same metadata surfaces as real Spanner:

- `GetDatabaseDdl`
- `INFORMATION_SCHEMA.ROLES`
- `databaseRoles.list`

Thanks!