Fix Jetpack conflict notice eligibility

[selul]

Summary

• add a Photon-specific compatibility whose should_load method checks Jetpack image Site Accelerator state
• register Jetpack in the generic conflict definitions only when that compatibility loads
• reuse the same should_load predicate for the dedicated Jetpack Photon dashboard conflict
• cover Photon enabled and disabled states with regression tests

Fixes #1130

Testing

• composer phpunit -- --filter=Test_(Jetpack|Plugin)_Conflicts (5 tests, 11 assertions)
• composer phpunit (251 tests, 1406 assertions)
• composer phpstan
• vendor/bin/phpcs --standard=phpcs.xml inc/compatibilities/jetpack_photon_compatibility.php inc/conflicts/conflicting_plugins.php inc/conflicts/jetpack_photon.php inc/manager.php tests/test-jetpack-conflicts.php
• local WP-CLI runtime check with Jetpack 15.8: Photon off reports no conflict; simulated Photon on loads the compatibility and reports both expected conflict surfaces

[pirate-bot]

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