fix(codegen/unreal): eliminate UObject memory leak in reducer event handlers

[brougkr]

Summary

• Fixes critical memory leak in Unreal C++ generated code where NewObject<UReducer>() is called for every reducer event but never cleaned up
• At 30Hz server tick rate (common for game servers), this creates ~30 orphaned UObjects per second, causing continuous memory growth
• Solution: Pass stack-allocated FArgs struct directly via new InvokeWithArgs() method instead of allocating UObjects

Problem

The generated ReducerEvent handler in SpacetimeDBClient.g.cpp creates a new UObject for every reducer event:

FArgs Args = ReducerEvent.Reducer.GetAs...();
UReducer* Reducer = NewObject<UReducer>();  // LEAK - never cleaned up!
Reducer->Param = Args.Param;
Reducers->Invoke...(Context, Reducer);

Unreal's garbage collector cannot keep up at high frequencies. Attempts to fix with MarkAsGarbage(), ConditionalBeginDestroy(), and transient package flags all failed because GC timing is unpredictable.

Solution

Eliminate UObject allocation entirely by passing FArgs directly:

FArgs Args = ReducerEvent.Reducer.GetAs...();
Reducers->Invoke...WithArgs(Context, Args);  // Zero allocation!

The original Invoke() methods that take UReducer* are preserved for backwards compatibility.

Changes

1. Header generation (~line 2515): Add InvokeWithArgs() method declaration
2. ReducerEvent handler (~line 3194): Call InvokeWithArgs() instead of creating UObject
3. Implementation (~line 3703): Add InvokeWithArgs() that extracts params from FArgs struct

Test Plan

• Verified with 30Hz TickGameScheduled reducer - UObject count remains stable
• Memory growth eliminated (was ~1000 UObjects per 14 seconds)
• Backwards compatibility preserved - existing Invoke() methods still work

Before/After

Before: OBJS count in Unreal stats continuously increasing, memory growing unbounded
After: UObjDelta=0 - no UObject growth from reducer events

[CLAassistant]

All committers have signed the CLA.

[JasonAtClockwork]

Thanks again for this PR I think it looks good but running into issues on my device to properly test it, I should have this confirmed on Monday!