SDK: Add contract binding for get_group (Savings contract)

[A6dulmalik]

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

Add an SDK binding for get_group, which retrieves full SavingsGroup metadata by group ID.

• Maps SavingsGroup struct including GroupStatus and Frequency enums
• Maps error variant: GroupNotFound
• Includes unit test asserting returned data matches created group
• Includes usage documentation

Reference: get_group in Savings.