fix(unreal): Replace FTickableGameObject with FTSTicker and add compressed message buffering

[brougkr]

Summary

This PR fixes two issues in the Unreal SDK:

1. FTickableGameObject initialization order bug - Replaced with FTSTicker for reliable tick registration
2. Fragmented compressed WebSocket message handling - Added buffering for messages that span multiple frames

Problem

Issue 1: FTickableGameObject

FTickableGameObject registers itself in its constructor BEFORE UDbConnectionBase's constructor body runs. Even with ETickableTickType::Never, UE's GENERATED_BODY() macro can interfere with base class initialization order, causing the default constructor to be called instead.

Issue 2: Fragmented Messages

Large compressed messages from SpacetimeDB can arrive fragmented across multiple WebSocket frames. The current implementation assumes complete messages arrive in single frames, causing decompression failures.

Solution

FTSTicker Replacement

• Removed FTickableGameObject inheritance
• Added FTSTicker::FDelegateHandle for manual tick registration
• Added destructor to clean up ticker registration
• Added OnTickerTick() method

Compressed Message Buffering

• Added CompressedBufferMutex to protect accumulation buffer
• Added IncompleteCompressedBuffer for fragment accumulation
• Added BufferedCompressionType to track compression across fragments
• Enhanced gzip validation to detect incomplete data and trigger buffering

Changes

• Public/Connection/DbConnectionBase.h - Class declaration changes, new members
• Private/Connection/DbConnectionBase.cpp - Implementation of buffering and ticker system

Testing

Tested in production Unreal 5.7 project with SpacetimeDB backend. Fixes previously observed:

• Random tick registration failures on connection
• Decompression errors on large subscription payloads