SDK: Add contract binding for get_round_contributions (Savings contract)

[A6dulmalik]

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

Add an SDK binding for get_round_contributions, which returns all contributions for a given round.

• Maps Contribution struct fields
• Includes unit test asserting contributions appear after contribute calls
• Includes usage documentation

Reference: get_round_contributions in Savings.