fix: sync relationship changes for users not in the local cache

[aashish00021]

This was reported and root-caused against stoatchat/for-web (the web client that consumes this SDK as a submodule), see stoatchat/for-web#1504 for the original bug report and repro ("friends aren't synced up correctly between clients"). The root cause lives here in the SDK's event handling, not in the web client, so the fix belongs in this repo.

UserRelationship events are re-dispatched internally as UserUpdate events. The UserUpdate handler only applies changes to a user that's already in the local collection it looks the user up with getOrPartial, which returns undefined for an unknown id when partials is disabled (the default this SDK's clients use).

So a relationship change made on another client accepting/receiving a friend request, blocking, unblocking, removing a friend is silently dropped here if this client has never cached that user before. It stays out of sync until the next full Ready resync (e.g. a page reload), which hydrates users unconditionally via getOrCreate.

Fix: ensure the user exists in the collection via getOrCreate before delegating to UserUpdate, using the data already included in the UserRelationship event payload the same data Ready already uses to hydrate users on initial sync, so no extra request is introduced.

Related to stoatchat/for-web#1504. This won't automatically close that issue, since for-web vendors this package as a submodule and will need its stoat.js reference updated after this lands.

How was this PR tested?

• prettier and eslint pass on the changed file
• tsc --noEmit passes
• Couldn't exercise this end-to-end against a live server/second
  client no way to trigger a real UserRelationship event
  without two authenticated sessions and an unknown-to-each-other
  user pair. Traced the fix against the Ready handler's
  equivalent getOrCreate path as the reference behavior.

Checklist:

• I have carefully read the contributing guidelines
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if applicable
• I have no unrelated changes in the PR
• I have confirmed that any new dependencies are strictly necessary
• I have written tests for new code (if applicable)
• I have followed naming conventions/patterns in the surrounding code

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