feat: implement IPolicyFilter in PolicyFilter

[seniorquico]

This fixes #385, implementing the IPolicyFilter interface in the PolicyFilter class so that it may be used with the filtered extension methods.

This also adds a set of very basic policy filtered tests for each of the built-in adapter types.

[hsluoyz]

@FAUST-BENCHOU plz review

[Copilot]

Pull Request Overview

This PR fixes issue #385 by implementing the IPolicyFilter interface in the PolicyFilter class, enabling its use with filtered extension methods. The change ensures that the PolicyFilter class conforms to the expected interface contract for policy filtering operations.

• Made PolicyFilter implement IPolicyFilter interface
• Added comprehensive test coverage for filtered policy operations across all built-in adapter types

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
Casbin/Persist/PolicyFilter.cs	Implements IPolicyFilter interface in PolicyFilter class
Casbin.UnitTests/PersistTests/AdapterTest.cs	Adds filtered policy tests for FileAdapter, StreamAdapter, and TextAdapter

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[github-actions]

🎉 This PR is included in version 2.15.0 🎉

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