feat: fix type check for FilteredAdapter using is operator

[Copilot]

The type check adapter.runtimeType == FilteredAdapter fails because FilteredAdapter is abstract—runtime types are concrete implementations like DefaultFilteredAdapter.

Changes

• loadFilteredPolicy() (L179): adapter.runtimeType == FilteredAdapter → adapter is FilteredAdapter
• isFiltered() (L198): adapter.runtimeType == FilteredAdapter → adapter is FilteredAdapter

// Before
if (adapter.runtimeType == FilteredAdapter) {
  filteredAdapter = (adapter as FilteredAdapter);
}

// After  
if (adapter is FilteredAdapter) {
  filteredAdapter = (adapter as FilteredAdapter);
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>wrong type check of adapter</issue_title>
<issue_description>In core_enforcer.dart, the type of the adapter is checked using adapter.runtimeType == FilteredAdapter, which would expect an exact match of FilteredAdapter, however FilteredAdapter is an abstract class so this will never evaluate to true.

This should be changed to: adapter is FilteredAdapter

Needs to be changed in two places:

https://github.com/casbin/dart-casbin/blob/9c72f3f5839d8cfbb456543a81891dc8c8556137/lib/src/core_enforcer.dart#L179

https://github.com/casbin/dart-casbin/blob/9c72f3f5839d8cfbb456543a81891dc8c8556137/lib/src/core_enforcer.dart#L198</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes wrong type check of adapter #57

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