feat: add getEscrowState method

[MarcusDavidG]

Summary

get_escrow_state(invoice_id) returns the current DisputeState (release_at, disputed, disputed_at) but there is no SDK method for it.

Method to add

async getEscrowState(invoiceId: number): Promise<{ releaseAt: number; disputed: boolean; disputedAt: number } | null>

Acceptance criteria

• Method added to SharpyClient
• Returns null if no escrow state exists for the invoice
• JSDoc included