Interface StellarTxPreparer

The single rpc.Server capability the prepare helpers need. Declared structurally instead of Pick<rpc.Server, 'prepareTransaction'> so the published .d.ts references stellar-sdk's concrete Transaction exports rather than its rpc namespace — the namespaced form trips dts-bundle-generator's external-type resolution under stellar-sdk v16. A real rpc.Server satisfies this structurally, so call sites are unchanged.