fix(Engine): don't crash on a duplicate key in a simple dictionary attribute

[alexwarren]

Summary

Follow-up found while investigating a user report about the RunCommand.aslx library (from EightOne, a popular community library): loading a game that includes it fails with Error: Argument_AddingDuplicateWithKey, "to" instead of the friendly "Failed to load game due to the following errors" message.

Root cause

RunCommand.aslx's EditorScriptsScriptsRunCommandPrePositions template lists the "to" mapping twice:

"on"=on;"in"=in;"from"=from;"to"=to;"with"=with;"about"=about;"to"=to

That string is parsed as a simplestringdictionary editor <validvalues> attribute by SimpleStringDictionaryLoader.Load (src/Engine/GameLoader/AttributeLoaders.cs), which called QuestDictionary<string>.Add(key, value) for every entry with no duplicate check. Dictionary.Add throws ArgumentException ("Argument_AddingDuplicateWithKey") on a repeated key, and nothing between there and the WASM boundary (WasmEditorBridge.Initialise) catches that specific exception type as a normal load error — it falls into the generic catch (Exception ex) instead, and because WasmEditor is AOT-trimmed, ex.Message loses its resource string and becomes the bare, untranslated resource key.

This is a bug in that particular library file's data (a copy-paste duplicate), not something introduced by quest itself — but the engine crashing ungracefully on malformed dictionary data, with a cryptic message, is a real robustness gap: any third-party library (or hand-edited game) with a duplicate key in a simplestringdictionary/simpleobjectdictionary attribute hits the same crash today.

Fix

SimpleStringDictionaryLoader and its sibling SimpleObjectDictionaryLoader (same copy-pasted parsing loop, same latent bug) now check for a duplicate key before adding and record a normal, friendly load error instead — exactly mirroring the existing handling for a missing = a few lines above in the same method. The game still fails to load (the dictionary is genuinely ambiguous), but with a clear, actionable message pointing at the offending element/attribute instead of a raw exception.

Test plan

• Added AttributeLoaderTests.cs with regression tests for both loaders: a duplicate key now produces a friendly WorldModel.Errors entry (Duplicate key '"to"' in dictionary element ...) instead of an uncaught exception — the assertion text matches the user's exact reported quoted-key format.
• dotnet build --configuration Release (full solution) and dotnet test --configuration Release (294+ tests) both pass.

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