feat: [SDK-4861] prepare Unity Asset Store UPM packages

[fadi-george]

Description

One Line Summary

Prepare the Core, Android, and iOS UPM packages for the Unity Asset Store multi-package product.

Details

Motivation

The Asset Store draft requires packages whose folders and manifest names match com.onesignal.unitysdk.core, com.onesignal.unitysdk.android, and com.onesignal.unitysdk.ios.

Scope

• Renames the three package folders and technical names while preserving Unity asset GUIDs.
• Sets the initial breaking cutover version to 6.0.0 and retains Unity 2022.3 as the minimum.
• Updates platform dependencies, local demo manifests, editor package paths, release workflows, and documentation.
• Keeps Full Usage exclusively in Core and removes the demo's scoped-registry requirement.
• Adds validation for package identity, metadata, version alignment, dependencies, and repository URLs.
• Does not change the public C# API.
• Legacy npm package deprecation remains separate from this change.

Testing

Unit testing

No unit tests were added because this change affects package layout and metadata rather than runtime behavior.

Manual testing

• python3 .github/scripts/validate-unity-distribution.py
• csharpier check .
• npm pack --dry-run --json for Core, Android, and iOS

Signed tarball validation, Publisher Portal upload, and clean-project installation checks require the OneSignal Unity organization session and remain release verification steps.

Affected code checklist

• Notifications
  • Display
  • Open
  • Push Processing
  • Confirm Deliveries
• Outcomes
• Sessions
• In-App Messaging
• REST API requests
• Public API changes

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
• Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• I have included test coverage for these changes, or explained why they are not needed
• All automated tests pass, or I explained why that is not possible
• I have personally tested this on my device, or explained why that is not possible

Final pass

• Code is as readable as possible.
• I have reviewed this PR myself, ensuring it meets each checklist item

Made with Cursor

[sherwinski]

Are we planning on simultaneously supporting UPM and NPM publishing? Why bump the package version to the next major version? Couldn't we support the same major version since this is the first time we're publishing to this registry?