fix: merge duplicate contract entries in convertOverridesToJSON

[jackchuma]

When Forge simulation produces multiple StateOverrideDecoded tuples for the same contract address, the previous code naively pushed one output entry per input tuple, yielding duplicate addresses in the stateOverride array. Replace the simple push loop with a two-phase aggregate-then-emit approach: phase 1 collects all storage slots into a Map keyed by lowercase address (last-wins on key collision), phase 2 emits one StateOverride entry per unique address in sorted order.

[cb-heimdall]

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