Support modifying data attachments using the /data command

[modmuss50]

This PR add a seperate set of logic for handling data attachment updates when using the /data command. This ensures that the correct events are fired, and the changes are synced to the client if required. It does so by using the higher level API's for updating data attachments as opposed to overwriting the list of attachments without progagating the updates as required.

Fixes #5275

[Copilot]

Pull request overview

Adds dedicated handling for /data-driven updates to Fabric data attachments so attachment callbacks and sync behavior stay consistent (fixing desync reported in #5275).

Changes:

• Wraps vanilla EntityDataAccessor#setData and BlockDataAccessor#setData to apply attachment changes through Fabric’s high-level attachment APIs.
• Skips the normal “read attachments from NBT” path while /data is being applied, and applies diffs via a new DataAccessorHandler.
• Adds unit tests plus a client gametest to validate /data modify updates sync to clients.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
fabric-data-attachment-api-v1/src/main/java/net/fabricmc/fabric/impl/attachment/DataAccessorHandler.java	New helper that diffs and applies attachment changes during /data operations.
fabric-data-attachment-api-v1/src/main/java/net/fabricmc/fabric/mixin/attachment/BlockDataAccessorMixin.java	Wraps block /data application to route attachment updates through DataAccessorHandler.
fabric-data-attachment-api-v1/src/main/java/net/fabricmc/fabric/mixin/attachment/EntityDataAccessorMixin.java	Wraps entity /data application to route attachment updates through DataAccessorHandler.
fabric-data-attachment-api-v1/src/main/java/net/fabricmc/fabric/mixin/attachment/AttachmentTargetsMixin.java	Avoids applying attachment NBT directly while a /data change is in progress.
fabric-data-attachment-api-v1/src/main/resources/fabric-data-attachment-api-v1.mixins.json	Registers the new accessor mixins.
fabric-data-attachment-api-v1/src/test/java/net/fabricmc/fabric/test/attachment/DataAccessorHandlerTests.java	New unit tests validating attachment add/update/remove behavior via DataAccessor set/merge.
fabric-data-attachment-api-v1/src/test/java/net/fabricmc/fabric/test/attachment/CommonAttachmentTests.java	Exposes mockRA() for reuse by new unit tests.
fabric-data-attachment-api-v1/src/testmodClient/java/net/fabricmc/fabric/test/attachment/client/gametest/SyncGametest.java	Extends client gametest to verify /data modify updates a synced attachment and reaches the client.
fabric-data-attachment-api-v1/src/testmod/java/net/fabricmc/fabric/test/attachment/AttachmentTestMod.java	Adds chicken initialization behavior for a synced item attachment in the testmod.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.