fix(server/state): ignore stale frame acknowledgements

[ThomasAlvenin]

Goal of this PR

Avoid creating an empty client frame state when an acknowledgement arrives after its frame has already been removed.

How is this PR achieving the goal

frameStates[frameIndex] inserts a default ClientEntityState when the ACK is for a frame that is no longer tracked.

Using find() lets us ignore those late ACKs without mutating the map. Existing frames still follow the same ACK path as before :p

This PR applies to the following area(s)

Server

Successfully tested on

Game builds: 3258

Platforms: Windows

Verified by replaying an ACK after its frame had already been removed.

Checklist

• Code compiles and has been tested successfully.
• Code explains itself well and/or is documented.
• My commit message explains what the changes do and what they are for.
• No extra compilation warnings are added by these changes.

Fixes issues

No linked issue.