fix: use correct is_enabled method

[chickenn00dle]

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Changes proposed in this Pull Request:

This fixes a fatal that resulted in calling the incorrect is_enabled method in #4541

How to test the changes in this Pull Request:

1. Visit Audience Management
2. Verify you don't get a fatal

Other information:

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[Copilot]

Pull request overview

Fixes a fatal error in Reader Activation (Audience Management) settings retrieval by using the correct integration-enabled check for the ESP integration, aligning with the integrations framework added in #4541.

Changes:

• Replace an instance is_enabled() call on the ESP integration with Reader_Activation\Integrations::is_enabled( 'esp' ) when resolving the sync_esp setting.

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[github-actions]

Hey @chickenn00dle, good job getting this PR merged! 🎉

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