[BUG] AbstractNodeDataChangedListener.onCommonChanged REFRESH stale-removal guarded by size> instead of diff — stale entries persist when |old|<=|new|

## Description
`onSelectorChanged` does `SelectorData selectorData = changed.stream().findFirst().orElseThrow(...); String configKeyPrefix = selectorData.getNamespaceId() + JOIN_POINT + changeData.getSelectorDataId() + JOIN_POINT;` — using `getNamespaceId()` directly with no null-default. Every sibling handler (`onAppAuthChanged:73-75`, `onPluginChanged:84-86`, `onMetaDataChanged:95-97`, `onRuleChanged:187-189`, `onProxySelectorChanged:276-278`, `onDiscoveryUpstreamChanged:287-289`) wraps the value with `StringUtils.defaultString(value.getNamespaceId(), SYS_DEFAULT_NAMESPACE_ID)` before building the prefix. If a `SelectorData` arrives with null `namespaceId`, the prefix becomes the literal `"null.selector."` instead of `".selector."`. The gateway-side nacos/apollo reader builds keys with the configured namespace id, so it reads `".selector."` and never sees the entries written under `"null.selector."` — selectors silently invisible to the gateway.

## Location
- `shenyu-admin-listener/shenyu-admin-listener-api/src/main/java/org/apache/shenyu/admin/listener/AbstractNodeDataChangedListener.java:176-177` (`onSelectorChanged`); compare siblings `:73-75,:84-86,:95-97,:187-189,:276-278,:287-289`

## Impact
Latent: any selector with a null `namespaceId` (partial config, legacy data, or a future code path that forgets to set it) is published under a wrong config key on nacos/apollo/polaris and is invisible to gateways, causing those routes to vanish while admin shows them as registered. The sibling methods defend against exactly this; selector alone does not.

## Suggested fix
Replace `selectorData.getNamespaceId()` at line 177 with `StringUtils.defaultString(selectorData.getNamespaceId(), SYS_DEFAULT_NAMESPACE_ID)` (and use the same streaming pattern as siblings to be consistent).

## Related existing
None — distinct from CLI-2 (#6664, client-side new-registrar never propagates namespaceId) and CLI-3 (rpcExt case mismatch). This is admin-side config-key construction for selector publish on node-based backends.

Start in shenyu-admin-listener/shenyu-admin-listener-api/src/main/java/org/apache/shenyu/admin/listener/AbstractNodeDataChangedListener.java at onSelectorChanged, then compare the namespace handling in the sibling handlers listed in the issue. The change is complete when null namespaceId values use SYS_DEFAULT_NAMESPACE_ID consistently and selector configuration keys match the gateway-side namespace keys.

Written by the indexing model from the issue text.