Regular users get "Cannot resolve the given keypair_resource_policy key" on the legacy GQL keypair_resource_policy query

## Symptom

When a regular user opens the WebUI, it fails with the following GraphQL error (surfaced through the Hive gateway, `serviceName: "graphene"`):

```json
{
"message": "Bad request. (Cannot resolve the given keypair_resource_policy key)",
"path": ["keypair_resource_policy"],
"extensions": {
"code": "backendai_generic_bad-request",
"serviceName": "graphene"
}
}
```

## Reproduction

Verified on `main` at `0ff5326d2f` (26.8.0rc1-364), local halfstack dev setup, DB migrations at head (`c58b0d3a9e14`).

1. Log in as a regular user (e.g. `user@lablup.com` from `fixtures/manager/example-users.json`).
1. Run the legacy GQL query the WebUI issues:

```

{ keypair_resource_policy(name: "default") { name max_concurrent_sessions } }

```

(e.g. `./bai gql '{ keypair_resource_policy(name: "default") { name max_concurrent_sessions } }'` in session mode)

1. The query fails with the error above.

## Observations

- The `default` row exists in `keypair_resource_policies`, and the same query succeeds for a superadmin (`admin@lablup.com`), returning the policy normally.
- The regular user's own keypair references `resource_policy = 'default'`, i.e. the user is querying the policy that applies to themselves.
- Setting `[manager.rbac] enforcement-enabled = false` in `manager.toml` and restarting the manager makes the same query succeed for the regular user; re-enabling the flag brings the error back. (Used as a local workaround only.)
- Reproduced identically before and after pulling the latest `main`.

## Expected

A regular user can read the keypair resource policy that applies to their own keypair (the WebUI depends on this query during normal use).

JIRA Issue: BA-7862

Reproduce the legacy keypair_resource_policy query with ./bai gql in session mode, using the regular user from fixtures/manager/example-users.json. Read the manager RBAC behavior configured through manager.toml and compare regular-user and superadmin results, with enforcement enabled and disabled. Done means a regular user can read the policy referenced by their own keypair and the WebUI query succeeds.

Written by the indexing model from the issue text.