[BUG] selector selectByIdSet/deleteByIds omit namespace_id — cross-namespace selector delete (+ cascade)

## Description
`SelectorServiceImpl.deleteByNamespaceId(List ids, String namespaceId)` calls `selectorMapper.selectByIdSet(new TreeSet<>(ids))` whose SQL is `WHERE id IN (...)` with no `namespace_id`, and the eventual `deleteByIds` is `DELETE FROM selector WHERE id IN (...)`. `namespaceId` is passed only to event publishing, never to SELECT or DELETE. Sibling `selectByQuery`/`updateSelective` enforce `namespace_id`, confirming bug.

Controller-exposed at `SelectorController:155`. The fetched selectors (unfiltered by namespace) also drive `unbindDiscovery` and the downstream `BatchSelectorDeletedEvent` rule cleanup, amplifying blast radius.

## Location
- `shenyu-admin/src/main/java/org/apache/shenyu/admin/service/impl/SelectorServiceImpl.java:297-303,633-636`
- `shenyu-admin/src/main/resources/mappers/selector-sqlmap.xml:80-88` (`selectByIdSet`), `465-471` (`deleteByIds`)
- `shenyu-admin/src/main/java/org/apache/shenyu/admin/controller/SelectorController.java:152-156`

## Impact
Cross-namespace selector deletion (and cascade unbindDiscovery/deleteSelector) — a user can tear down selectors in another namespace, removing routing configuration and bound discovery bindings. Requires authenticated admin with `system:plugin:delete`.

## Suggested fix
Add `AND namespace_id = #{namespaceId}` to `selectByIdSet` and `deleteByIds`, or filter ids via a namespace-scoped select first.

## Related existing
Distinct from H-2 (rule) and N2 (#6615) — different mapper file and operation.

Start at SelectorController.java:152-156 and trace SelectorServiceImpl.java:297-303 and 633-636 into selector-sqlmap.xml entries 80-88 and 465-471. Verify that selector lookup and deletion remain namespace-scoped, including the unbindDiscovery and BatchSelectorDeletedEvent cleanup paths. Done means an authenticated deletion cannot affect selectors or bindings outside the requested namespace.

Written by the indexing model from the issue text.