Switching models in the pricing editor kept the previous model's tiers and prices in the expression panel: TieredPricingEditor seeds its internal visual/raw state only on mount, and the initRef guard never re-ran on prop changes, so only the model name updated.
Bump a reload token in the same effect that seeds billingExpr and use it as the editor's key, so a freshly loaded model remounts the editor and re-parses its expression. The token changes in lockstep with billingExpr, and user edits (which only touch state) do not trigger it.
Closes #5750
