Fix race condition in undesired models state by making it observable

[Copilot]

_currentModelObs reads undesired models synchronously via _undesiredModelsManager.isUndesiredModelId(), but that state is updated asynchronously in setCurrentModelId. This causes stale reads when the observable recomputes before async updates complete.

Changes

• Make undesired models state observable: Added ISettableObservable<string[]> to UndesiredModels.Manager, exposed via modelsObservable getter
• Wire observable into dependency chain: _currentModelObs now reads undesiredModelIds from the observable
• Pass state explicitly to _pickModel: Accepts undesiredModelIds: string[] parameter instead of reading from manager

this._currentModelObs = derived<Model, void>((reader) => {
    return this._pickModel({
        preferredModelName: this._preferredModelNameObs.read(reader),
        models: this._modelsObs.read(reader),
        undesiredModelIds: this._undesiredModelsManager.modelsObservable.read(reader),
    });
}).recomputeInitiallyAndOnChange(this._store);

The observable updates after persisting to global state, ensuring the dependency chain recomputes with consistent data.

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