fix(account): hasWorkspacePermission returned true for missing grants

[MichaelUray]

Summary

AccountDB.hasWorkspacePermission reports every permission as granted.

Both implementations do

const result = await this.workspacePermission.findOne({ workspaceUuid, accountUuid, permission })
return result !== undefined

but findOne resolves to null when nothing matches (PostgresDbCollection.findOne returns … ?? null, the Mongo driver returns null), so the comparison is always true.

This PR compares against null instead (result != null) in the Postgres and the Mongo implementation and adds tests for the missing and the existing grant in both.

Impact

No upstream code path consumes hasWorkspacePermission today (the import permission UI uses getWorkspacePermissions), so nothing is exposed right now — but any server-side check built on this method (for example a per-workspace "may create projects" grant) would silently allow everyone.

Testing

• npx jest src/__tests__/postgres.test.ts src/__tests__/mongo.test.ts in server/account — new tests fail before the change, pass after.
• Verified on a self-hosted deployment where a permission check is built on this method: before the fix an account without the grant was allowed, after the fix it is denied, and granting/revoking works as expected.