SDK: Add contract binding for get_round_deadline (Savings contract)

[A6dulmalik]

Implementation path: sdk/src/savings/get-round-deadline.ts (tests: sdk/src/savings/__tests__/get-round-deadline.test.ts)

Add an SDK binding for get_round_deadline, which returns the contribution deadline timestamp for a specific round.

• Maps input params: group_id, round
• Maps return type (u64 Unix timestamp → number)
• Maps error variant: GroupNotFound (also returned for invalid round number)
• Includes unit tests for valid round and out-of-range round
• Includes usage documentation

Reference: get_round_deadline in Savings.