adsConversionTrackingIntent feature flag #13463

Feature Description

The Ads conversion tracking intent is built across many issues, and each of them needs the same flag so its work stays hidden until the whole flow is ready. This issue adds adsConversionTrackingIntent to the list, so PHP can check it with Feature_Flags::enabled( 'adsConversionTrackingIntent' ) and React components can check it with useFeature( 'adsConversionTrackingIntent' ).

The flag is only declared here. Nothing reads it yet.

Link to the design doc: https://docs.google.com/document/d/1-8aikH7EzELWZCFuava3izGfhUVPiOFE3Oww4czjTW0/edit?tab=t.0

Do not alter or remove anything below. The following sections will be managed by moderators only.

Acceptance criteria

• adsConversionTrackingIntent is listed in feature-flags.json.

Implementation Brief

• In feature-flags.json:
  • Add "adsConversionTrackingIntent" as a new entry in the array.

Test Coverage

• N/A

QA Brief

• No QA needed; can be moved directly to Approval.

Changelog entry

• Add the adsConversionTrackingIntent feature flag.