…elds (#15865)

Fixes `contains` operator on hasMany `select` fields in PostgreSQL. This
was a regression introduced in #15671.

## The problem

Given a collection with a hasMany `select` field:

```typescript
{
  slug: 'users',
  fields: [
    {
      name: 'roles',
      type: 'select',
      hasMany: true,
      options: ['user', 'admin', 'editor'],
    },
  ],
}
```

Payload stores this across two PostgreSQL tables. For a user created
with `roles: ['admin', 'editor']`:

**`users`**
| id | created_at | updated_at |
|----|------------|------------|
| 1  | 2026-03-05 | 2026-03-05 |

**`users_roles`**
| id | parent_id | value    | order |
|----|-----------|----------|-------|
| 1  | 1         | `admin`  | 1     |
| 2  | 1         | `editor` | 2     |

The `value` column is a PostgreSQL **enum type**, not a `varchar`.

When querying `{ roles: { contains: 'admin' } }`, Payload generated:

```sql
SELECT DISTINCT "users"."id"
FROM "users"
LEFT JOIN "users_roles"
  ON "users"."id" = "users_roles"."parent_id"
WHERE "users_roles"."value" ILIKE '%admin%'
-- ❌ PostgreSQL error: operator does not exist: enum_users_roles ~~* unknown
```

`ILIKE` does not work on PostgreSQL enum types.

## The fix

The LEFT JOIN already flattens the array into individual rows:

| users.id | users_roles.value |
|----------|-------------------|
| 1        | `admin`           |
| 1        | `editor`          |

So "contains admin" just means "has a row where value equals admin". The
fix converts `contains` to `equals` for hasMany `select` fields, the
same way it already works for hasMany `relationship` and `upload`
fields:

```sql
SELECT DISTINCT "users"."id"
FROM "users"
LEFT JOIN "users_roles"
  ON "users"."id" = "users_roles"."parent_id"
WHERE "users_roles"."value" = 'admin'
-- ✅ Matches row 1, returns user id=1
```

## Why hasMany `text` fields are different

HasMany `text` fields store values as `varchar`, not as an enum. `ILIKE`
works fine on `varchar` columns, so substring matching (`contains:
'adm'` matching `'admin'`) is valid there. Whether it's _desirable_ is a
problem separate from this PR, being discussed internally
[here](https://payloadcms.slack.com/archives/C049BR3QBHC/p1772680331359859)