Fix compile on UE 5.8: FJsonObject keys are now FSharedString

[javidmedina]

What

FJsonObject::Values is keyed by UE::FSharedString in UE 5.8 rather than FString (see Runtime/Json/Public/Dom/JsonObject.h). Any loop over Values that passes Pair.Key where an FString is expected stops compiling. Ten call sites fail:

AnimationBlueprintUtils.cpp(930,16):        error C2665
MCPToolBase.h(173,15):                      error C2664
MCPToolBase.h(175,13):                      error C2665
MCPTool_AnimBlueprintModify.cpp(778,14):    error C2665
MCPTool_BlueprintModify.cpp(401,23):        error C2664
MCPTool_BlueprintModify.cpp(548,23):        error C2664
MCPTool_Material.cpp(544,10):               error C2664
MCPTool_Material.cpp(569,10):               error C2664
MCPTool_Material.cpp(587,10):               error C2664
MCPTool_SetProperty.cpp(379,35):            error C2039   (.ToUpper() on FSharedString)

How

Each site is wrapped as FString(*Pair.Key).

This is deliberately version-agnostic rather than #if ENGINE_MINOR_VERSION-gated: TSharedString::operator* returns const TCHAR*, and so does FString::operator*, so the same expression compiles against both the 5.8 shared-string storage and the older FString storage. No preprocessor branching, no behaviour change.

Not included

I left EngineVersion in UnrealClaude.uplugin at 5.7.0 so this stays a pure compile fix and doesn't change what the launcher reports for existing users. Anyone building for 5.8 will want to bump that locally.

Testing

Built against UE 5.8, Win64, Development Editor. Results:

• Plugin module compiles clean
• Module loads at PostEngineInit
• Registered 28 MCP tools, MCP Server started on http://localhost:3000
• GET /mcp/status returns {"status":"running","port":3000,"toolCount":28,...}

Not regression-tested against 5.7 — I don't have a 5.7 install — but the change is a no-op there by construction.